Unearth 6U (6U: Return this card from your graveyard to play. It gains haste. Exile it at the beginning of the next end step or if it would leave the battlefield. Unearth only as a Sorcery.)
This is an example of the Unearth ability, taken from Kederekt Leviathan. The amount of mana required to Unearth varies from creature to creature. If the Unearthed creature would leave play in any way, ie. Returned to hand, put on top of the library etc, then it is Exiled instead.