Cassia the Lewd

Flying, Lifelink(Damage this creature deals is
converted into life)

Once per turn, you may pay {w}{w}{w}: Tap target creature with {b} in it's mana cost. It doesn't untap
during it's controller's untap step. Add an Impurity counter
to Cassia.

Impurity counters can be sacrificed to generate one {b} mana.