Sadako Yamamura, Onryō

Arvinox, the Mind Flail isn't a creature unless
you control three or more permanents you don’t
own.
At the beginning of your end step, exile the
bottom card of each opponent's 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.