Oberyn Martell, the Red Viper

First strike, wither (it deals damage to creatures
in the form of -1/-1 counters.)
At the beginning of your end step, if Oberyn
Martell, the Red Viper is in your graveyard,
put a -1/-1 counter on each creature with a
-1/-1 counter on it.
When Oberyn Martell, the Red Viper dies, put
a -1/-1 counter on target creature.