Return up to one target creature card from your graveyard to your hand. Create a 1/1 red Mercenary creature token with "{T}: Target creature you control gets +1/+0 until end of turn. Activate only as a sorcery."
You don't have to choose a target for Mourner's Surprise. However, if you do, and that creature card is an illegal target at the time Mourner's Surprise tries to resolve, it won't resolve and none of its effects will happen. You won't create a Mercenary token.