Woodlurker Mimic

Creature — Shapeshifter

Whenever you cast a spell that's both black and green, this creature has base power and toughness 4/5 until end of turn and gains wither until end of turn. (It deals damage to creatures in the form of -1/-1 counters.)

A mimic's disguise fools its victims just long enough to draw them within range of its venomous tentacles.