Mordekaiser, Iron Revanent

When Mordekaiser enters the battlefield; the battlefield becomes the realm of the undead until Mordekaiser leaves the battlefield.
Creatures can't enter the realm of the undead.
Mordekaiser gains +X/+X where X is the greatest power among creatures in the realm.
Mordekaiser returns to it's owner's hand when
it is the only creature on the battlefield.