As a player of Warcraft III expansion pack and the undead race, there are a few corrections needed to be made the death knight's abilities:

Death coil: Still does the heal/damage mentioned above. To pick a great chunk out of an enemy hero, a lich is usually used, so that a coil closely followed by a frost nova hits the enemy hero. This is generally called a nuke. Death knight is chosen first for his coil because undead do NOT have a tier 1 healing move.

Death pact: buffed considerably, but still not picked much by players. It now returns 100/200/300% of the sacrificed health.

Unholy aura: chosen as the second skill of the death knight, sometimes even as the first to get a few nice early surrounds on night elf heroes. Gives 0.5/1/1.5 hp regen/sec, and gives 10/20/30% bonus movement speed. Ghouls with frenzy and this aura will make them quite alarming even at tier 3.

Resurrect: Yeah. Crazy ultimate of Death knight. BUT. As coil/aura are quite imbalanced when both at level 3, so some opt for resurrect at level 7. The 6 corpses raised are now invulnerable to all attacks/magic, and last 40 seconds.