Gengar the Nightmare

Gengar isnt a creature unless you control three
or more permanents you dont own.
Nightmare - At the beginning of your end step,
exile the bottom card of each opponents library
face down. For as long as those cards remain
exiled, you may look at them, you may cast
permanent spells from among them, and you may
spend mana as though it were mana of any color
to cast those spells.