Most of Necro Mutants and some others have this ability.
- Curse reduces damage dealt, and Drain is a percentage of that. So a 20% Curse will reduce the total amount of HP drained by 20%.
- Shields don't reduce it since it's based on damage dealt, not on the amount of HP your opponent loses.
- Taking effect after Retaliate means that a mutant with Drain can get KOed BEFORE the drain actually happens.
For most mutants this ability starts at 25% and increases to 35% at level 25.
On mutants with Spread Attacks, Drain is applied per hit, thus it can drain all three enemies.