Briserec, the Shatterer

When Briserec enters the battlefield, exile fifteen
cards from the top of your library.
Exile five or ten cards from the top of your
library facedown: You may play a random card
this turn that was exiled by Briserec. If you exiled
ten cards to pay for this ability, instead choose
one of the cards exiled by Briserec, then an
opponent chooses one, then choose one of these
two at random. You may play that card this turn. Activate this ability only once each turn.