Drain Life

Drain Life is a channeled level 9 warlock spell that damages a targeted enemy, draining their health to restore the warlock's own.

Talents

 * Inevitable Demise (Affliction)

PvP Talents

 * Essence Drain
 * Rot and Decay (Affliction)

Shadowlands

 * Accrued Vitality (Endurance Conduit)
 * Claw of Endereth (Runecarving Power)
 * Soul Rot (Night Fae Covenant Ability)

Tips and Tactics

 * Drain Life should only be situationally used due to its high mana cost. Overuse of this spell will cause the warlock to run out of mana.
 * Running oom will affect the warlock's ability to deal damage as most warlock spells have a mana cost. There are no mana restoring abilities available to warlocks.

Background
This appears to be the Life Drain ability that Dark Rangers used in Warcraft III. This ability absorbs the life essence of a target enemy unit and gives it to the Dark Ranger.

Lore
Warlocks could leech life energy and power their own magical abilities. The warlocks' death/mortal coil spell is a variation of the Drain Life spell and is one of the necromantic abilities warlocks share with necromancers.

Draining the life energy of creatures, plants, and people with magic ages them.

Trivia

 * Drain Life is a basic Warlock spell in Hearthstone. The flavor text reads: ""I've just sucked one year of your life away.""
 * Drain Life is one of Gul'dan's Basic Abilities in Heroes of the Storm.
 * A drain life skill rune exists in Diablo.
 * In  Classic, mechanical mobs are immune to Drain Life.