Dormen, Envoy of Tarkir

When Dormen, Envoy of Tarkir enters the battlefield, reveal cards from the top of your library until you reveal a Dragon creature card.
Put that card into your hand and the rest into your graveyard.
Whenever a Dragon with power 5 or greater enters the battlefield under your control, exile Dormen, Envoy of Tarkir, then return him to the battlefield transformed under his owner's control.