Curse of Rotbugs

Enchant player
At the beginning of enchanted player's upkeep,
put a -1/-1 counter on a creature they control
without a -1/-1 counter on it if able. If you don't,
put a -1/-1 counter on a random creature they
control. Then you make a 1/1 black Bug creature
token for each creature enchanted player controls
with a -1/-1 counter on it.