Wikia

Drain Life
Drain Life 40 yd range 1.0% of base mana Channeled Drains the life from the target, causing X Shadow damage and restoring 2% of the caster's total health every 1 sec. Lasts 6 sec. Affliction Soulburn:Healing increased by 50%.
Usable by
Class	Warlock
Properties
Type	Offensive, Defensive
School	Shadow
Cooldown	(GCD 1.5 sec)
Level required	7
Related debuff
Drain Life Suffering X Shadow damage every 1 seconds. Restoring 2% of maximum health to the caster when Drain Life deals damage. Duration: 6 seconds

Drain Life is a core warlock ability learned at level seven. It allows warlocks to damage enemies, draining their life to restore their own.

Modified by

Because Drain Life deals damage and heals, it receives a 50% penalty from +damage gear. The total bonus is then distributed amongst the five ticks of the spell. For example - if you have +100 spell damage, each tick of Drain Life will transfer +10 HP from the target ((100*0.5) / 5).
Drain Life combined with [Life Tap] can allow a Warlock to act as a tank.

Patch 2.3.0 (13-Nov-2007): The amount healed is reduced when the warlock is affected by healing reducing effects (e.g. Mortal Strike, Wounding Poison).