Hapatra, Vizier of Poisons

Whenever Hapatra, Vizier of Poisons deals combat damage to a player, you may put a -1/-1 counter on target creature.
Whenever you put one or more -1/-1 counters on a creature, create a 1/1 green Snake creature token with deathtouch.

Combo

Devoted Druid

{T}: Add {G}.
Put a -1/-1 counter on Devoted Druid: Untap Devoted Druid.

Quillspike

{B/G}, Remove a -1/-1 counter from a creature you control: Quillspike gets +3/+3 until end of turn.