Yerlasdermas, Queen of Poison

Deathtouch

{b}{b}: Whenever a creature you control with deathtouch deals damage to another creature this turn, prevent all combat damage dealt to those creatures. After the next combat step, put X -1/-1 counters on any number of target creatures whos damage was prevented this way, where X is the damage prevented by creatures you control.

Yerlasdermas, Queen of Poison gets +1/+1 for each -1/-1 counter among creatures your opponents control.