Lagratha, the Twisted Crone

Lagratha, the Twisted Crone enters the battlefield with three -1/-1 counters on it.
Whenever you cast a black spell, you may move one or more -1/-1 counters from Lagratha, the Twisted Crone onto another target creature. If you don't, put a -1/-1 counter on Lagratha, the Twisted Crone.