Marissa, Designer Poisoner

At the beginning of your upkeep, put a -1/-1 counter on Marissa.
Whenever one or more -1/-1 counters are put on a creature you control, put that many -1/-1 counters
on target creature you don’t control.
At the beginning of your end step, remove a -1/-1 counter from target creature you control.

“If one is to create better poisons, one must not be squeamish when picking their test subjects.”