Arrok, Ferocious Swampstalker

Menace, wither (A source with wither deals damage
to creatures in the form of -1/-1 counters.)
Whenever Arrok, Ferocious Swampstalker deals
combat damage to a player, put a wither counter
on target creature you control.
{b/g}{b/g}{b/g}{b/g}, {t}: Proliferate. Activate only as a
sorcery and only once each turn.