Enchant creature When Dark Favor enters the battlefield, you lose 1 life. Enchanted creature gets +3/+1.
When he began to curse what he held holy, his strength grew unrivaled.
Official Rulings for Dark Favor
9/22/2011 : If the creature targeted by Dark Favor is an illegal target when Dark Favor tries to resolve, Dark Favor will be countered. It won't enter the battlefield and its enters-the-battlefield ability won't trigger.