Tylara, Penitent Lorekeeper

Whenever this card enters the battlefield, look
at the top three cards of target opponent's
library; exile one of them face down,
then put the rest of them into their graveyard.
For as long as that card remains exiled, you may
look at it, you may cast it, and you may pay mana
as if it were any type to cast that spell.
{w/b}, Sacrifice a creature: exile Tylara,
Pentitent Lorekeeper, return it under your
control at the beginning of your next upkeep.