Grimgrin, Corpse-Born enters the battlefield tapped and doesn't untap during your untap step.
Sacrifice another creature: Untap Grimgrin and put a +1/+1 counter on it.
Whenever Grimgrin attacks, destroy target creature defending player controls, then put a +1/+1 counter on Grimgrin.
9/22/2011 If the targeted creature is an illegal target by the time Grimgrin’s last ability resolves, the entire ability doesn’t resolve and none of its effects will occur. You won’t put a +1/+1 counter on Grimgrin.
9/22/2011 If the defending player controls no creatures when Grimgrin attacks, the last ability will be removed from the stack and have no effect.
7/1/2013 If Grimgrin’s last ability resolves, but the targeted creature isn’t destroyed (perhaps because it regenerated or has indestructible), you’ll still put a +1/+1 on Grimgrin.