Drain Power is a skill located in the Siphoning (which can be found in the Nightblade skill tree). Drain Power is a base skill, and can be morphed into Power Extraction or Sap Essence.
Drain Power deals the following types of damage: Magic Damage. Which means that it gets buffed by the following blue champion point trees: Elemental Expert, Elf Born, Master at Arms, Spell Erosion. Also the following red champion point trees protect against this skill: Elemental Defender, Iron Clad.