My Succubus' Vampirism

Crunch:
low level: buff
mid level: heal and buff
lower-high level: heal and buff + skill steal, 1 slot, single use then lost forever
upper-high level: heal and buff + skill steal, 3 slots at max, semi-permanent but only 20% efficacy of the true user and degrades after each use

Fluff:
The succubus has a certain body part that she (and her minion in varying level) can use to absorb the life force of her victims.