When Demonlord of Ashmouth enters the battlefield, 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.)
5/1/2012 Demonlord of Ashmouth’s enters-the-battlefield ability will trigger no matter how it entered the battlefield, including because of its undying ability.
5/1/2012 You don’t have to sacrifice another creature, even if you control one. You may choose to exile Demonlord of Ashmouth.