When you deal damage with your Corruption spell, you are instantly healed for 40% of the damage done. In addition, the damage done by your Corruption, Seed of Corruption and Unstable Affliction damage over time effects is increased by 5%.
Gorefiend tightened his fist and blood suddenly burst from the hapless orc’s nose, eyes, and mouth. Gorefiend had already turned away by this point, having lost interest in tormenting the annoyance. “Dark magic!” one of the Shadowmoon warriors shouted, grabbing up the axe beside him. “Kill the warlocks before they can afflict any more of us!” he bellowed, and his fellows responded by readying themselves as well.”[1]
Siphon Life is a warlocktalent in the Affliction tree. It adds a healing effect to [Corruption] that has a 25/50% chance to heal you for 2% of your total health. It is unlocked only after spending two points in [Soul Siphon].
Warlocks have the ability to siphon life from their victims. This ability appears to have been used by the warlock-death knight Teron Gorefiend to torment a member of the Shadowmoon clan.
In the RPG[]
This section contains information from the Warcraft RPG which is considered non-canon.
Gouts of blood erupt from a victim's eyes, nose and mouth. The streams spin and twist into a single, larger, rope-like stream as they bridge the gap between the target and the caster. The caster opens his mouth and draws in the stream, gaining vitality as the victim weakens.[2]
Patch 4.0.1 (2010-10-12): Redesigned as a tier 2 talent with 2 ranks (was tier 4 with 1 rank). When you deal damage with your [Corruption] spell, you have a 25/50% chance to be healed for 2% of your total health. Now requires [Soul Siphon].
Old: When you deal damage with your Corruption spell, you are instantly healed for 40% of the damage done. In addition, the damage done by your Corruption, Seed of Corruption and Unstable Affliction damage over time effects is increased by 5%.
Patch 3.1.0 (2009-04-14): The Siphon Life spell has been removed. Siphon Life now causes your Corruption spell to instantly heal you for 40% of the damage done.
[Corruption]
[Soul Siphon].
Patch 5.0.4 (2012-08-28): Removed. Replaced with Glyph of Siphon Life. 
[Shadow Mastery]. 
