Kel'thuzad by ArchRanger

Tier 1:
A Welcomed Chill(Q): Adds a medium HoT to Frost Armor
Blight Patch: Active 45 seconds, place a patch of blight that provides vision for 20 seconds. Cannot be attacked
Conjurer's Pursuit
Necromancer's Pursuit: Every 7 globes increases the mana gained from minions by 1 point for the Curse of Naxxramas trait

Tier 2:
Healing Ward
Frozen Mithril (Q): Increases the shields by 15%
Absorbing Chains(W): Chained allies will absorb 10% of their basic attack damage as HP for the chain duration
Undead Pact (W): Chains now give allied heroes 20% increased mana regen for the chain duration

Tier 3:
Mana Hungry(E): Increases the mana destroyed to 15%
Volatile Explosion (E): Increases the AoE damage done by 150% 
Overwhelming Aura(trait2): Increases the HP regen by Unholy Aura by 75% but increases the mana cost by 100%
Reflective Glare (Q): Increases the attack speed debuff to 20%

Heroics

Tier 5:
The Shadows Beckon (trait1): Increases the mana from minions to 8 and heroes to 20%
Scourge Foundry (W): Adds a second charge and reduces cooldown by 3 seconds, Chains don't stack
Winter Armory (Q): Adds a second charge
Necromancer's Secret (Q): Casting Frost Armor on an allied hero also applies a Frost Armor for Kel'thuzad

Tier 6:
Frost Bite: Basic attacks now have a stacking slow effect of 5%, up to 20%. Lasts 3 seconds since the last hit
Soul Drain: Basic attacks against enemy heroes now gives 4 mana per hit
Mana Leech (E): The mana destroyed gets returned to Kel'thuzad
Blood for Blood 

Tier 7:
Shadow Enchantments (R1): Increases damage to 7% of enemy heroes' HP and 25 per second, slow increased to 20% and allies no long affected by slow
Instant Fissure (R2): Removes the 3 second delay and reduces cooldown by 40 seconds
Storm Shield
Bolt of the Storm
Dark Ambition (trait2): Increases movement speed of allies under Unholy Aura by 10%

Kel'thuzad doesn't function like your normal support/assassin hybrid like Kharazim does. Instead, he is about buffing his allies until the right moment appears where he drops an offensive heroic that secures the fight for his team. He has no form of direct healing without the support of talents but rather focuses on HP regen and buffing it for his teammates.

One of his more interesting abilities is Detonate Mana, a mana burn ability. It doesn't do much damage or take much mana and is on a very long cool down but a well placed E on an assassin or support could deny them their heroics. Using this move on Morales or Kaelthas could cripple their contribution to a team fight (especially the drawn out ones where you can use two Detonates). Malfurion is a strong counter to this ability and it isn't very effective when the enemies are near healing wells but adds another layer of strategy to the dark Kel'thuzad.