Shadowborn Demon

Oracle Text

Flying

When Shadowborn Demon enters the battlefield, destroy target non-Demon creature.

At the beginning of your upkeep, if there are fewer than six creature cards in your graveyard, sacrifice a creature.

Card Rulings

7/1/2013 Shadowborn Demon’s enters-the-battlefield ability is mandatory. If you control the only non-Demon creature, you must choose it as the target.
7/1/2013 The last ability checks whether you have fewer than six creature cards in your graveyard when it would trigger. If you have six or more, it won’t trigger at all. The ability will check again when it tries to resolve. If at that time you have six or more, the ability won’t do anything.