Passive: Once pr second a stacking siphon debuff is put on an attacker that hit the necromancer.
active: siphon debuffed opponent(s), erasing the debuff stack, and put the signet on recharge.
The risk is that to put marks out the necromancer have to be in combat and taking damage. And once those debuffs are “cashed in”, the necro have no heal for the duration of the signet recharge.