I've been playing Kel'Thuzad a lot since his release, and found many things that I have not seen mentioned anywhere. I wanted to share some of these things and therefore I decided to make this guide. This is the first guide I've written, so please bear with me.
With that out of the way I can give you a proper Welcome!
I honestly don't play ranked much at all (I don't even get past placements some seasons), but I've gotten to Diamond.

Pros

• Has a high damage combo that can 100-0 not one, but two squishies
• Has a lot of CC
• Deals extremely high damage once he completes his Master of the Cold Dark quest
• The game's deadliest sieger

Cons

• His damage is very telegraphed, allowing people with good shields, spell armor and invulnerability (like Ice Block) to tank his combo easily. In addition, people that can become Unstoppable can easily dodge the bulk of his damage.
• No Mobility
• Only skillshot abilities
• Deals lackluster damage until Master of the Cold Dark completes
• He's hard to play
• Usually awful in 1v1 fights
• Can be countered hard, by mobile heroes and heroes that can Cleanse allies or make them Invulnerable.

Kel'Thuzad is a good choice when:

• Your team has a heavy frontline
• The enemy team has low-mobility heroes like Arthas and Kael'thas

You may want to refrain picking Kel'Thuzad when:

• Your team lacks the frontline needed to protect Kel'Thuzad.
• The enemy team has high-mobility assassins like Genji, as it is hard to hit them as they close the gap to kill you.
• The enemy support has good options to Cleanse or shield allies, like Uther or Medivh
• Early in a draft, as Kel'Thuzad can be counterpicked hard. Preferably you want the enemy team support to already be locked in.

(Passive) Master of the Cold Dark This quest gives Kel'Thuzad his damage. The +75% spell power from completion means he deals almost twice as much damage after the quest is completed. Opposed to other people's powerspike at lvl 10 when ultimates come into play, completing this quest is Kel'Thuzad's powerspike. At 15 stacks, you also get reduced cooldowns as an appetizer. These are nice for extended fights, but not much of a powerspike. It is preferable to complete the quest before level 10, as then you can bring your full combo to bear with your ultimate right at level 10. Not having the quest completed at that time means everyone gets their level 10 powerspike, whereas you don't get your powerspike until the quest completes. Everyone tends to become a lot better at killing people after level 10, and teams also roam together consistently at that point, both of which gives you less chances to earn stacks. Completing the quest around lvl 7 is hard, but feels great; then you're in great position to terrorize people with the lvl 7 talent, before ultimates come into play. That being said, don't give up on Kel'Thuzad when you don't complete your quest before the game ends. Earning stacks can be hard if you can't get in range without dieing, and Kel'Thuzad can get counterpicked hard. I personally lost count of how many games I didn't finish the quest before game's end, still having had such a game recently. Needless to say, you want to complete this as fast as possible. To this end, you want to engage in as many fights as possible. Kel'Thuzad has a lot to gain from lvl 1 fights; try to teamfight with your team at lvl 1. Throw Chains of Kel'Thuzad at enemies when safe to do so, even when you can't combo them. If you are in lane with only a single enemy, you can often not do much more to get stacks than throw chains at them whenever it is off cooldown. To earn stacks faster, seek out lanes with more than one enemy in it, so you can do a Chains of Kel'Thuzad + Frost Nova combo on two of them for 4 stacks. Just be careful to not get engaged on when you're outnumbered. Alternatively, ganking a lane will cause the enemy laner to run for the cover of his buildings; take advantage of this by chaining him to one of them. Just try to avoid staying in sololanes for extended periods of time.

(Q) Death and Decay Death and Decay is your most spammable skill. After a short channel, Kel'Thuzad throws out a green blob that explodes into a small puddle when it hits an enemy. The damage on this is quite good for the low cooldown and manacost, so spam it when it is safe to do so. Even if spammed, it will usually be off cooldown when you do land a combo. Throw Death and Decay at minions and buildings whenever you can do so safely (it has more range than turrets). It can be hard to see for enemies in exactly which direction this will go, therefore you can hit with it surprisingly often. When it explodes, the puddle will mostly spawn in front of the target you hit. In order to hit the most minions/mercs, aim for the ones in the back. Against a minion wave, you can hit most by aiming at the caster minion from the side. One issue with Death and Decay is that unlike all of Kel'Thuzad's other skills, it can get blocked by minions or buildings. Especially when you chain someone to the other side of a structure, the structure will block you from landing Death and Decay as a followup, unless it is a gate. For this reason, prefer using a gate over other buildings as a target for Chains of Kel'Thuzad.

(W) Frost Nova This AOE root is best used as a setup for a Death and Decay and a Shadow Fissure. If you only hit with the edge, it only slows, which is not nearly as good (and also doesn't give you a passive stack). Note that if you hit with the center, it will first root, and after the root ends it slows for the remainder of the slow duration. It has too much of a delay to land the root without any setup, and even the slow can be dodged on reaction. To land the root, use it as a followup for other cc (like Chains of Kel'Thuzad). If I miss Chains of Kel'Thuzad, I have a habit of throwing it out on its own (it has a lower cooldown than Chains of Kel'Thuzad), but if your allies have CC it may be useful to hang on to it.

(E) Chains of Kel'Thuzad This is probably Kel'Thuzad's most signature ability. You throw out chains in a target direction. If you hit an enemy hero or enemy building, the chains latch on to them for a few seconds. At this point, it can be reactivated to fire the chains again, but this time from the first target's position. If you manage to hit a second enemy hero or enemy building, it pulls the two of them together, stunning heroes briefly afterwards. Note that you cannot hit friendly buildings. If it hits two heroes it pulls both of them to halfway between them. However it cannot pull heroes over walls, and will only pull up to the wall. If one of the heroes is unstoppable, it will pull the other one all the way towards the unstoppable hero. This is similar to hitting a hero and a building. If you hit a hero and a building, it will pull the hero all the way towards the building. Hitting two buildings will do pretty much nothing, doing little more than putting up some creepy decorations for a second. Chains of Kel'Thuzad is amazing when there is something around you can chain a hero to, but if there nothing else to latch Chains of Kel'Thuzad to, it deals awful damage and leaves you without a reliable way to land the rest of Kel'Thuzad's abilities. For this reason Chains of Kel'Thuzad, and by extension Kel'Thuzad himself, is awful in 1v1 situations, unless you have Glacial Spike. Important here is that the further targets get pulled, the longer it takes for them to arrive, and the longer it takes before they are able to move again. This matters a lot for a Frost Nova followup (described in the combo section). It can be hard to hit this ability, but it comes out fast. Therefore enemies cannot dodge it on reaction, only by good prediction. This makes it Kel'Thuzad's best combo opener. A good tactic is to first aim for people on the enemy frontline. Aiming for the backline immediately is often too dangerous, as this ability has a relatively short range. When you hit someone on the frontline, you can use his position to reach targets further away. However, if you hit you want to be in range to hit Frost Nova between both of your targets. Basically, you can either aim for the second-closest person to follow up more safely, or aim for the backline to try and hit a priority target as well. The ability to chain from structures makes Kel'Thuzad a deadly sieger. The range you get from hitting a building and then chaining from there means enemies usually cannot defend structures you're in range of without risking death. Therefore, if you have a pushing objective and don't need to defend, you should push with it unless vastly outnumbered. As a Kel'Thuzad player, always leave walls alive as a target for chains.

(R) Frost Blast Frost Blast is Kel'Thuzad's first ultimate choice. You throw a Pyroblast-esque projectile towards a target of your choice. When it (finally) arrives, the target and anyone near it gets rooted. It deals solid damage to secondary targets, but only minor damage to the primary target. Unlike Pyroblast you don't need to channel before use, but the target still gets a lot of time to prepare for impact. Sadly the projectile is often too slow to save yourself with if someone is busy killing you. If they're not actively killing someone, they usually run away really far before it hits, because they don't want to be rooted near you or your team. Therefore the best result you can expect in most fights is to cause someone on the enemy team to run out of the fight, and leave their team outnumbered. If someone does let himself be rooted near you (likely only if they were already at least slowed), you can follow up with Frost Nova and Death and Decay. Instead of trying to open up with Frost Blast, you can use it after the rest of your combo to keep enemies locked down for longer, as well as deal more damage to them. However, it doesn't deal nearly as much damage as Kel'Thuzad's other ultimate, Shadow Fissure. Frost Blast is good versus a team with many (3+) frontline heroes, because melee heroes often cluster when they collapse on someone. This is a good opportunity to land a multi-hero Frost Blast. Having high followup damage on your team also helps get a lot more value out of Frost Blast, as it and Frost Nova in sequence can root people for 2.5 seconds (or 3 seconds if you have Blighted Frost). Ultimately Shadow Fissure is my preferred pick for its damage potential. I honestly don't have too much experience with Frost Blast since I usually pick Shadow Fissure.

(R) Shadow Fissure Shadow Fissure is Kel'Thuzad's second ultimate choice. You target a small area, anywhere on the map, and after 1.5 seconds the ground erupts to deal heavy damage to enemy heroes. It also has a measly cooldown of 20 seconds. The fact that this is a heavy damage global ultimate may make you giddy in excitement, the fact that it has a low cooldown even more so. However, before you start throwing it out willy-nilly, you should realise it hits only a small area with a 1.5 second delay. It is easy to dodge Shadow Fissure, and anyone able to move and not asleep will do so. Therefore, you almost always want to use Shadow Fissure as a followup to CC, whether that is your own combo, or a stun from a teammate. It has a 20 second cooldown, so you should use it whenever someone on the enemy team gets stunned and you notice. If an enemy is not CC'd, you may still want to use Shadow Fissure. For example when people are immobilizing themselves using important abilities like Ravenous Spirit or Siege Mode: you'll force them to either stop channelling or eat the damage. Shadow Fissure is also great to interrupt people capturing an objective, like a Cursed Hollow tribute. You can also use Shadow Fissure when someone on the enemy team is getting chased down. Drop it in their path and they will have to change direction to dodge. You or your teammates may be able to use this detour to catch up and secure the kill. Feel free to use Shadow Fissure on people not likely to pay attention, for example on someone recalling. They will likely be hiding in the fog of war, but a blind cast may snipe you a kill. A final note is that Shadow Fissure only deals damage to heroes, so you can't use it to poke down objectives, buildings or clear a minion wave. Shadow Fissure is so much more useful than Frost Blast IMHO that I kinda make the assumption in the rest of the guide that you pick Shadow Fissure as your ultimate.

I feel like this deserves its own section. Basically, you want to land a Frost Nova, so you can follow it up with Death and Decay and Shadow Fissure. However, Frost Nova is hard to land by itself. The best opener for Kel'Thuzad is usually Chains of Kel'Thuzad. You can follow up the pull from chains by rooting both targets with Frost Nova. If you hit Chains of Kel'Thuzad, you can then follow up with Frost Nova, Shadow Fissure and Death and Decay. The full casting order is thus:
-> -> -> ->
Knowing where exactly to put down Frost Nova in this case takes experience; I really can't describe it better than that other than saying "In the middle between them".

However, if the targets of Chains of Kel'Thuzad are close together, they can actually walk out of a followup Frost Nova before it lands. Even if the two heroes are further apart, heroes with a blink can still dodge the root. To deny enemies the ability to blink or move out after getting hit by chains, you can actually swap the order of casting the second cast of Chains of Kel'Thuzad with Frost Nova so Frost Nova lands sooner. Then the cast order becomes
-> ->
However, if you are chaining a hero to a building (or to Glacial Spike) from long range, Frost Nova actually lands too soon if cast before the second cast of Chains of Kel'Thuzad, because the pulling animation is longer. In these cases you should stick to the order
-> ->
Try doing the chains -> nova -> chains combo and you will figure out this combo is hard; I recommend not to bother with this alternative order until comfortable playing Kel'Thuzad.

---

The Plaguelands - This talent changes your poison puddle from murloc-sized to dehaka-sized. In fights, this gives the Q better AOE damage and zone denial. While this talent doesn't add too much burst damage, it does give a lot of sustained damage. With this Kel'Thuzad gets much better waveclear, merc power, objective damage and siege damage. I like the utility of it a lot, so I prefer this talent as the level 1 pick if I don't want either other talent.

---

Blighted Frost - If you want to stop people from blinking out of your combo before the grand finale, you pick this talent. By default, Frost Nova has a 1 second root, but Shadow Fissure has a 1.5 second delay. This means everyone with a blink, and many others with mobility skills can dodge the Fissure even if Nova lands. If 2 people on the enemy team have such skills, you may want this to ensure their demise. Extra damage on a center hit with Frost Nova sure is appreciated, but all lvl 1 talents deal extra damage one way or another; it's not what you pick this talent for.

---

Barbed Chains - Gives the most combo damage out of all the level 1 talents. If you land chains, you're probably going to follow up with a combo, so this talent makes both deal more damage. Your teammates get to benefit from this too. If you wanna deal with tanks by trying to burst them, this talent helps you nuke hardest out of all the level 1 talents. For squishies this is not needed to kill them unless they are healed and you have no backup from your team. Pick this talent when you want to combo multiple beefy people like tanks and bruisers.

---

Strip Shields - Makes Chains of Kel'Thuzad deal bonus damage to shields. You should pick this talent if there is heavy shielding on the enemy team, like 2 of Tassadar, Zarya or Artanis. Note that most shielding abilities will likely be cast AFTER you cast Chains of Kel'Thuzad, to save whoever is about to get comboed. In such a case Strip Shields gives no benefit. The shields you get from hitting with chains is nice, but they're not substantial enough to warrant picking this talent if the enemy team has no shields.
FYI: the damage from Strip Shields is not amplified by Barbed Chains' 125% increased Chains of Kel'Thuzad damage.

---

Phylactery of Kel'Thuzad - After gathering 12 regen globes, you heal 10% of all spell damage you deal. This allows you to heal up easily, especially outside of teamfights. Just drop a Death and Decay on a minion wave to heal half of your hp. However, the healing usually doesn't help much when there is someone in your face killing you.
In addition, you can reset the quest to respawn instantly if you're dead.
Likely you will only be able to respawn once or twice per game using this talent; that being said, respawning after a big teamfight can secure or save the game late in the game. Therefore I choose Phylactery of Kel'Thuzad when there is no big argument for either other talent on this tier.

---

Armor of the Archlich - You get 50 physical armor at the push of a button. Great when an Illidan or Tracer is in your face. Take this when the enemy has a lot of physical damage, especially on divey heroes like Tracer. The 50% damage reduction is usually the difference between dieing and forcing them to back off from your team after trying to kill you. When you activate Armor of the Archlich, you also slow and deal some damage around you. This is not really worth activating it for, but the slow will prevent someone who was just trying to kill you from running away after noticing you got 50 armor and are not going to die. Usually just one melee autoattacker is all it takes to make me pick this talent, it is just that good.

---

Accelerated Decay - Makes Death and Decay deal more damage the longer enemies stay in it. Sounds good on paper, but the extra damage is not quite amazing enough to compare to the amazing utility Glacial Spike or Chilling Touch brings. Both Glacial Spike and Chilling Touch make it easier to combo someone, but Accelerated Decay doesn't bring that to the table. Therefore I can't recommend picking it unless your team is loaded with CC and you're running a Death and Decay build.

---

Glacial Spike - Spawns some terrain that you can attach your Chains of Kel'Thuzad to. Sounds gimmicky, but this is actually amazing for Kel'Thuzad. Kel'Thuzad's weakness is that in a 1v1 situation, you can't stun with Chains of Kel'Thuzad, and therefore can't hit the rest of his kit. However, if you have Glacial Spike, you have a way to stun with Chains of Kel'Thuzad in a 1v1 situation. A quick sidenote is that Glacial Spike deals damage when it expires and explodes, but this damage is so poor that it is generally not worth accounting for.

You can go about chaining someone to Glacial Spike in two ways: Either you use Chains of Kel'Thuzad on Glacial Spike first and then try to hit a hero. The alternative is that you first aim at a hero with Chains of Kel'Thuzad, and then aim for Glacial Spike.
If you hit Glacial Spike first, then you can hit someone with Chains of Kel'Thuzad at twice its range by casting Glacial Spike between you and your target. Doing so also pulls the target towards you and your team, allowing your team to focus him. However, because Glacial Spike takes a second to spawn, enemies will usually run out of range before it spawns and you can use Chains of Kel'Thuzad on it. This makes it harder to catch people out of position a la Hook; However, in teamfights you can use this range extension trick to stay safer while you try and pick someone off.
The other option of hitting the hero with Chains of Kel'Thuzad first is generally more helpful up close. In this case you don't need the extra range. If you get dived for example you can cast Chains of Kel'Thuzad on whoever's diving you and spawn Glacial Spike right next to him, and chain him to it as it spawns. Without extra movement abilities he can't run far enough from the Glacial Spike to avoid getting chained to it. An advantage of casting Glacial Spike only after you hit Chains of Kel'Thuzad is that you avoid it going on a 30-second cooldown should you miss.

Another use of Glacial Spike is blocking enemies' movement. Glacial Spike is blocking terrain, and can block enemies running through a chokepoint for 4 seconds. It can block chokepoints surprisingly wide, because heroes often cannot squeeze between the gap left between Glacial Spike and other terrain. You can block enemies' escape or help yourself escape. To help yourself escape, cast it just in front of you, so it spawns just after you run over. However, unlike Zombie Wall, you can't get rid of Glacial Spike early. Therefore be careful to not screw over any allies you wanted to help, they probably won't be able to take it lightly.

---

Chilling Touch - Makes your autoattack deal splash damage and slow every 8 seconds. Note that this autoattack deals spell damage and therefore benefits from your Master of the Cold Dark spell damage boost. The slow may help you land a Frost Nova, followed by the rest of your stuff; however I personally find it easier to land a combo with Glacial Spike. Unlike Glacial Spike, Chilling Touch does increase your damage somewhat, which is nice.
Chilling Touch can be a solid pick if you don't need the extra range that Glacial Spike brings, but I personally like Glacial Spike so much that I hardly ever pick Chilling Touch.

I have already discussed ultimates in the Abilities section. My preferred pick is Shadow Fissure

---

Icy Grasp - Extends Frost Nova's slow from 2.5 seconds to 4.5 seconds. 4.5 seconds is a long time to be slowed, and will make it hard for anyone that got hit to get away or try to get revenge. This effect is only useful if enemies survive or you can't fully combo them, but that is also the case with Chain-Link. One advantage of Icy Grasp is that it can also be useful if you chain a hero to a structure, whereas Chain-Link won't trigger in such a case. Unless enemies are really not going to die when you combo them (in which case I pick Chain-Link), Icy Grasp is my preferred pick for this tier.

---

Chains of Ice - On paper a slow with such a short duration really doesn't sound useful. However, it helps a lot to land a Chains of Kel'Thuzad -> Frost Nova combo by making it harder for enemies to run out of the Frost Nova after getting chained. That being said, it is possible to cast Chains of Kel'Thuzad and Frost Nova in such a way that dodging Frost Nova after getting chained is impossible even without this talent (described in the "The Kel'Thuzad combo" section). This makes it kinda feel like a learning talent (or more negatively said: a crutch talent) for Kel'Thuzad. Despite this, I strongly recommend you to make use of this talent for three reasons:
First, Kel'Thuzad can be hard to play, and it is better to pick such a talent if you like it than get frustrated playing Kel'Thuzad and quit on him.
Second, the other two talents on this tier aren't the most crucial talents, and you can easily manage without them.
Third, Chains of Ice is only available from level 13. This means that if you want to do well with Kel'Thuzad, you will learn to play without Chains of Ice anyway. This makes it much easier to try out the other talents without being too attached to Chains of Ice.

---

Chain-Link - Lower Chains of Kel'Thuzad's cooldown by 4 seconds when you chain two heroes together. Note that Chain-Link does not lower cooldown if you chain someone to a structure or Glacial Spike. If you have played with Kel'Thuzad, you will generally think that if Kel'Thuzad manages to pull off a 2-man combo, they usually both die and he already won the teamfight for his team. Therefore Chain-Link would be more of a win-more talent. This is generally true, unless the enemy team has heroes that are really good at nullifying a Kel'Thuzad combo, like Uther with Divine Shield and Cleanse. If you are having trouble swaying a fight in your favour despite being able to setup a 2-man combo, then you may wanna pick Chain-Link to be able to try again sooner.

---

Arcane Echoes - Reduces Death and Decay cooldown by 1.25 seconds for each hero you hit with its explosion. Since Death and Decay's radius is not very large, you will likely only hit one hero. You also get no cooldown reduction from landing Death and Decay coming from The Damned Return. However, The Plaguelands does help you score hits on more heroes with the size increase. Don't expect to take large chunks out of Death and Decay's cooldown. I only feel like Arcane Echoes is useful for a dedicated Death and Decay build (and I don't want to recommend such a build in serious games). Theoretically in such a build you can use the cooldown reduction from Arcane Echoes to be able to reapply Accelerated Decay before it wears off. However, a better way to increase Death and Decay's lingering damage is to use Hungering Cold.

---

Hungering Cold - Makes enemies hit by the center of Frost Nova take bonus damage every time they get damaged again by Kel'Thuzad. This bonus damage is about equal to Death and Decay's lingering damage. Since Death and Decay has a lot of damage ticks, the damage from Hungering Cold really adds up to an amount comparable to casting another ability. Therefore Hungering Cold is a standard pick in my build.

---

Power of Icecrown - Gives you up to 30% additional spell power depending on how many times you CC enemies. Power of Icecrown is easy to stack as Kel'Thuzad, because he has a lot of CC with Frost Nova, Chains of Kel'Thuzad and optionally Frost Blast. Note that the talents Armor of the Archlich, Chilling Touch, Chains of Ice also slow enemies and can thus give you additional stacks. For example, landing a 2-man Chains of Kel'Thuzad when you have Chains of Ice gives you 4 stacks of Power of Icecrown instantly. That being said, your Master of the Cold Dark quest also gives you 75% spell power. Because of how spell power stacks, you only get (100% + 75% + 30%) / (100% + 75%) = 117% of your normal damage at max stacks. A 17% damage increase isn't as much as Hungering Cold gives. However, using Power of Icecrown is easier, as it doesn't rely on landing a Frost Nova -> Death and Decay combo. If you like to melt face with Kel'Thuzad's comboes, you probably want Hungering Cold. If you also want to increase the rest of Kel'Thuzad's damage, you may want Power of Icecrown.

---

Deathchill - Makes Frost Blast slow after the root for 3 seconds. In addition, if a hero hit dies before either the root or the slow afterwards expires, he explodes again with a chance to hit teammates. I honestly don't pick Frost Blast much, and therefore Deathchill even less. Therefore I really don't have much experience with Deathchill.
4.5 seconds is a good-sized window for someone to die to be able to trigger secondary explosions. However, Deathchill doesn't give more damage to help killing someone in the first place, and Frost Blast already has lower damage than Shadow Fissure. I can only imagine Deathchill to be hilarious versus The Lost Vikings or Murky, except they both have ways to dodge Frost Blast. Also, before you start getting very excited, Clones or Mirror Images don't give you extra explosions.
I feel like Deathchill is mainly useful if your team has a way to keep enemies clustered with something like Grav-O-Bomb 3000 or Mosh Pit.

---

Might of the Scourge - Instant reset when you hit someone with Shadow Fissure. Sounds amazing, but you do have to realise that Might of the Scourge almost never hits by itself. Therefore pick if your team has a lot of CC. Another reason to pick this is against Sgt. Hammer or someone with a long channeled ultimate. You can repeatedly cast Might of the Scourge on them to force them to stop channeling and move. With allied CC, you may be able to pull off a Shadow Fissure -> Frost Nova -> Shadow Fissure combo. I generally pick Might of the Scourge when I have no big reason to pick either Shifting Malice or The Damned Return.

---

Shifting Malice - Okay, I lied when I said that Kel'Thuzad doesn't have any mobility, this one lvl 20 talent is the only mobility tool he has. However, I hope you still agree with me that doesn't have the mobility options to really chase or flee.
I pick this when I need to escape from people like Genji or am otherwise getting caught out of position a lot. In such cases, Shifting Malice is Kel'Thuzad's only tool to survive. It actually deals decent damage and resets on kill, so you may want to Shifting Malice into someone who is really low health. However, do note that Shifting Malice is not the fastest movement ability, so you may still get killed mid-dash.

---

The Damned Return - Spawns a shade that mimics your Death and Decay casts. The shade's Death and Decay benefits from all talents except for Arcane Echoes. The shade's cooldown is equal to its duration, so you can have a 100% uptime.
Hitting someone with both Kel'Thuzad's and The Damned Return's Death and Decay deals high damage and is a great way to chunk squishies and tanks alike. If you use The Damned Return during a combo, especially if you also have Hungering Cold, you truly deal stupendous amounts of damage.
You can poke at enemies or buildings safely by casting The Damned Return at range, and then casting Death and Decay from there. It doesn't dissapear when you get far away, so you can put it down, walk far away and poke. This is my preferred pick at lvl 20 if I don't need Shifting Malice and have The Plaguelands.

• Avoid 1v1 unless you have Glacial Spike, even a 1v2 is more advantageous for you than a 1v1 if you've got nothing to chain your enemy to (but that doesn't mean it's usually a good idea).
  
• Kel'Thuzad is not a bad lane soaker due to decent waveclear, but you probably wanna prefer staying with the team to stack your Master of the Cold Dark faster. Staying with the support is usually a solid plan.
  
• With Master of the Cold Dark completed, Kel'Thuzad is pretty good at taking mercs or pushing, do so if the occasion arises.
  
• Autoattack people, it is the only non-skillshot you have. Don't bother trying to hit low-health people with Death and Decay if you can finish him with several autoattacks.
  
• Kel'Thuzad really has trouble hitting high-mobility heroes, and can do pretty much nothing to kill them. Your best choice when one goes for you is to huddle your teammates and hope they are capable of getting rid of them.
  
• Refrain from locking in Kel'Thuzad early in draft and leave him till most enemy picks are locked in, especially the support. While non-supports may have abilities like Ice Block that save themselves, supports have abilities like Crystal Aegis that will save any one hero getting comboed, denying you a kill every fight. Should you be the one banning and are in doubt what to ban, my ban of choice is Genji.

You've made it to the end. I hope this piece of writing has been useful to you.
If you've got something on your mind about the guide, I am open to feedback.