Elemental Mastery

Oracle Text

Enchant creature

Enchanted creature has "T: Create X 1/1 red Elemental creature tokens with haste, where X is this creature's power. Exile them at the beginning of the next end step."

Card Rulings

5/1/2008 The enchanted creature’s power is checked at the time the ability resolves. If the enchanted creature has left the battlefield by then, its last known information is used.
5/1/2008 The tokens will be exiled in the End step even if Elemental Mastery or the enchanted creature has left the battlefield by then.
5/1/2008 If the ability is activated after the current turn’s End step has begun, the tokens won’t be exiled until the next turn’s End step.