Soul Siphon

This one: Gain the Soul Siphon ability: Increases health drained or damage….

• Soul Siphon · spell 9373 · Shadow · Drains 2 health every 2 sec. from an enemy and its nearest ally…
• Soul Siphon · spell 17804 · Increases the damage done or health drained by your Drain Life… · Warlock talent
• Soul Siphon · spell 18371 · Shadow
• Soul Siphon (Rank 1) · spell 7290 · Rank 1 · Shadow · Drains 10 health over 10 sec. from an enemy and its nearest ally…

Values as each build had them, not tested in play.

1. Forever 1.60.1.69913vs Era 1.15.9.69722
   Description, Era 1.15.9.69722

   Gain the Soul Siphon ability: Causes your Drain Soul to to deal damage 4 times faster and increases the amount drained by your Drain Life and Drain Soul spells by an additional 7% for each of your Warlock Shadow effects afflicting the target, up to a maximum of 21% additional effect. When Drain Soul is cast on a target below 21% health, it instead gains 51% per effect, up to a maximum of 153%. In addition, your Drain Soul can now trigger your Nightfall talent.

   Description, Forever 1.60.1.69913

   Gain the Soul Siphon ability: Increases health drained or damage done by your Drain Life, Drain Soul, and Wrack spells by 20%.