Drains the target's soul, causing (83.6% of Spell power) Shadow damage over 4 sec. If the target dies and yields experience or honor, one Soul Shard is energized.
When Drain Soul deals damage, it causes all of your other damage over time spells on the target to instantly deal 40% of their normal damage.
Deals (20.9% of Spell power) Shadow damage every 1 sec and causes all of the Warlock's other damage over time spells to instantly deal 40% of their normal damage.
The truest agony goes deeper than mere flesh and bone.[1]
Drain Soul is a channeled Afflictionwarlock spell that steals the soul from the enemy, dealing damage that is doubled if the target is at or below 20% health.
Patch 5.4.0 (2013-09-10): No longer energizes Soul Shards from the pets and guardians of players, and now deals 34% less damage. If other periodic Affliction damage effects are triggered by Drain Soul, they now deal 60% of their normal damage (down from 100%).
Patch 5.0.4 (2012-08-28): Now an Affliction specialization ability. Duration reduced from 15 to 12 sec, and causes double damage below 20% health instead of 25%. Now also causes all other periodic Affliction damage effects to instantly deal 100% of their normal periodic damage when the target is below 20% health.
Patch 4.0.3a (2010-11-23): Damage has been reduced by approximately 12%.
Patch 4.0.1 (2010-10-12): This spell now refreshes 3 soul shards.
Patch 3.3.0 (2009-12-08): This spell now deals 4 times the normal damage for all ranks. Previously it was only ranks 6 and above.
Patch 3.1.0 (2009-04-14): Each time Drain Soul deals damage to a target which can grant experience, it now has a chance to generate a [Soul Shard].
Patch 3.0.2 (2008-10-14): Will now cause four times the damage if the target is at or below 25% health.
Patch 1.5.0 (2005-06-07): Honorable kills will now generate a Soul Shard when Drain Soul is used.
[Soul Siphon], 
[Everlasting Affliction], 
[Pandemic]
[Grimoire of Sacrifice]: Increases the damage of your Drain Soul by 20%.
