Omnath, Locus of None

When Omnath enters, return target Elemental creature card from your graveyard to your hand.

Whenever another nontoken monocolored Elemental enters under your control, note that creature's color, then, if you've noted one of each color this way, add {w}{u}{b}{r}{g}, exile Omnath,
then return him to the battlefield transformed under its owner's control.