Gul'dan, hands down the most evil mortal in the Warcraft universe, has arrived in the Nexus. Gul'dan is the figure that the WoW Warlock class is based on, and so you're going to see some familiar abilities if you've played much WoW. For a little lore backstory, Gul'dan was originally a Shaman of the Shadowmoon Clan. When the Burning Legion decided to use the Orcs to exterminate the Draenei, the demon Kil'jaeden appeared as the ghost of the Shadowmoon chieftain's wife. But it was in the chieftain's apprentice that Kil'jaeden found his most useful resource. Gul'dan wanted power, pure and simple, and Kil'jaeden felt no need to disguise himself when appearing before the orc that would come to be called Darkness Incarnate. Gul'dan happily served the Legion, convincing the Orcs to drink demon blood, binding them to serve the agents of chaos. Guilty of just about every crime imaginable, Gul'dan finally got himself killed when trying to wrest the power of the fallen Titan Sargeras from his Avatar's tomb. Thanks to our alternate-universe-hopping misadventures in Warlords of Draenor, a new version of Gul'dan is back, leading the Legion on a massive invasion of Azeroth.
Gul'dan is a ranged assassin hero, with strong sustained damage and powerful self healing. Let's go over his abilities.
D: Life Tap:
Gul'dan does not passively regenerate mana. Instead, you'll need to sacrifice health to regenerate your mana. He does seem to have a decently large health pool to help with this.
Q: Fel Flame:
On a short cooldown, Gul'dan sends out a wave of green fire, starting narrow and then growing wide when it reaches its max distance. This will definitely cover an entire minion wave, and it does moderate damage.
W: Drain Life:
On a somewhat longer cooldown, you can target a hero, minion, or mercenary (I don't think it works on buildings) and deal constant damage over a few seconds while you heal considerably. This is going to be a very important move for you to compensate for the Life Taps you'll need for mana, but obviously it also works to heal you from damage that enemies have inflicted. This ability doesn't cost mana.
You target an area that looks like three circles in a row. The three areas explode with shadows, one after the other, starting with the one closest to you. Enemies within these areas will get a small damage-over-time effect on them. However, if they are hit by multiple explosions, such as if they are running away from you as you cast this, they will get multiple stacks of the DoT, which adds up to a pretty significant amount.
R: Rain of Chaos:
You channel for several seconds, standing still while felfire meteors rain from the sky over a large area. When each meteor lands, it does a large amount of damage to anyone nearby.
Targeting an area, this will send enemy heroes running in all directions for a couple seconds.
Gul'dan is relatively tough, which is good given that you're going to be damaging yourself constantly to sustain your attacks. The upshot of this is that Gul'dan is actually quite sustainable out in the field. Use Drain Life whenever you can, but try to hit targets that will stick around and not die any time soon. The heal is the same regardless of what you're hitting, but if the target dies, the effect ends.
You're also going to want to be careful with positioning, as Gul'dan doesn't have any escape abilities. Pairing with a healer will make Life Tap a more forgiving mechanic, but make sure they know not to heal you when you could just as easily use Drain Life - in other words, outside of intense group fights, healers should probably just let you take care of yourself.
Corruption is also tricky to use optimally, but it's also fantastic against enemies that are fleeing, as they tend to have low health, and your DoT is the perfect thing to finish them off.