Gul'Dan was an orcish warlock who betrayed his master and corrupted the world of the orcs, Draenor, in exchange for great power. He opened the Dark Portal and started the First war against the humans of the world of Azeroth, which resulted in the destruction of the city of Stormwind and the corruption of most of the orcs.
Gul'Dan holds a powerful magic, but his hunger for power is his great weakness. He died, ambushed by a horde of demons, while he was exploring the Tomb of Sargeras, seeking to unlock the Dark Titan's powers for himself.

Gul'Dan is a ranged, DPS assasin, classified as a mage hero. He has more HP than the other mages, but lacks mobility.

His Trait, life tap, has both a passive and an active effect. As passive, you don't restore mana over time as other heroes do. When activated, part of your current life is lost and you restore 25% of your mana.

Gul'Dan's Q, Fel flame, is his primary tool for dealing damage in the early game. It deals damage in a cone in front of our character and has a very low cooldown, which means it can be spammed (be careful, though, because it comes at a great mana cost. I recommend to cast 2 or 3, and then use life tap).

Drain life is Gul'Dan's W. When casted upon a living objective (you can't cast it on forts, but you can in Gazlowe's turrets) it deals damage and restores Gul'Dan's health. It's a channeled ability, so be careful when to use it, because you can be interrupted and easily killed. One remarkable thing about Drain life is that the damage and restoration of health are two separate effects. If you use it on an invulnerable character, it won't deal any damage, but it'll heal you.

Gul'Dan's E is Corruption, and it's what our build focuses around, making it your main source of damage in the late game. When casted, it summons three pools of dark energy in a straight line. When one of these pools hits an enemy, it'll make him take damage over time, stacking up to three times. Your main objective will be to push enemies into your corruption pools. In order to do this, cast fel flame to harass your opponents, and when they retire from combat to restore their health, you cast the first pool right under their feet, making them run into the other two.

Gul'Dan first heroic ability is Horrify. When casted, after a short delay, Gul'Dan summons an explosion of dark magic that produces fear in the enemies and makes them run away from the central point of the explosion. It's a great tool both for engaging (separating their team and make some enemies run right into your allies) and disengaging, forcing your opponents to run away from you.

Rain of destruction is Gul'Dan's second heroic. It consist on a 6s channeling in which you summon a lot of meteors that fall randomly in a very big area and deal damage. It's very poor compared to Horrify and you should truly never pick it.

Level 1
At level one you must pick Echoed corruption. It consists on a quest: you have to hit 40 targets with your E. When completed, Corruption summons another three pools that go backwards when the first three have finished.

Level 4
We have two options at level 4 when we're playing Gul'Dan. If you're not very used to life tap, I recommend you to pick Improved life tap, which will make your trait easier to use. But if you are accustomed to your trait you should go for consume soul. It allows you to instantly kill an enemy minion and restore part of your health. It's great for fighting in lane and useful to avoid some burst damage, and it can also be combined with life tap to fulfill both your HP and Mana before, during, or after a teamfight.

Level 7
We do also have two options at level 7. If you don't have a support because you're playing quickmatch, Hunger for power will grant you a lot of damage without any payback. If you have a support, you can still go for hunger for power, but I highly encourage you to pick Bound by shadow. This talent makes your Q reduce the cooldown of your E when it hits an enemy hero. It's very useful because it combines your two main sources of damage and allows you to cast corruption more often.

Level 10
We pick Horrify at level 10, as I explained in the first chapter of this guide.

Level 13

Healthstone should always be your pick at level 13. When activated, it restores 25% of your maximum HP. Perfect for some emergency life taps or to avoind burst damage.

Level 16

Without doubts, Unstable affliction is the best talent of Gul'Dan, apart from his heroic ability. It causes Corruption pools to deal damage when they hit. Then, If an enemy walks into three of these pools, he receives a massive amount of damage, apart from a huge DOT effect. When combined with the level 1 talent, Echoed corruption, it can burst down half healt of an entire team.

Level 20

Again, we have two viable choices for Gul'Dan's level 20. First of all, we have Haunt, which increases the duration of Horrify's fear and makes enemies vulnerable for as long as the fear is active. It's great in teams based on picking up kills and then disengaging. The other option we have is Demonic circle. It gives us two new spells: cast demonic circle makes Gul'Dan create a green circle where he's standing in, and Teleport to the demonic circle allows him to instantly appear where the demonic circle was casted, consuming it. It's great against teams with hard engage or great chase potetial.