From WC3:
Life Drain
Absorbs the life essence of a target enemy unit by taking hit points from it every second and giving them to the Dark Ranger. Also applies the current rank of the Black Arrow debuff to the target.
Come on Blizzard get your act together. Life Drain should replace Rapid Fire for Dark Rangers. It’s so obvious. If it would just replace the Animation and SFX Effects, i would already be happy.