Warlock · Affliction · tier 5 · from level 30

Siphon Life

Rank 1/1

Transfers 11 health from the target to the caster every 3 sec. Lasts 30 sec.

Doesn't change the damage or healing of any Warlock spell.

Averages, counting crits and misses, for a level 60 Warlock with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.