Imprisoned Guard

{b}{b}{b}: Monstrosity 1 (If this permanent isn't monstrous, put a +1/+1 counter on it and it becomes monstrous.)
Whenever Imprisoned Guard becomes
monstrous, sacrifice a creature and destroy target creature.
At the beginning of your end step, Imprisoned
Guard loses monstrous.
Imprisoned Guard can't attack or block if it isn't monstrous.