
Flying
When this creature enters, exile it unless you sacrifice another creature.
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
Illustrated by Lucas Graciano
Released on May 4, 2012
You don't have to sacrifice another creature, even if you control one. You may choose to exile Demonlord of Ashmouth.
Demonlord of Ashmouth's "enters" ability will trigger no matter how it entered, including because of its undying ability.