Court of Locthwain

When Court of Locthwain enters the battlefield, you become the monarch.

At the beginning of your upkeep, exile the top card
of target opponent’s library. You may play that card
for as long as it remains exiled and mana of any type
can be spent to cast it. If you’re the monarch, until
end of turn, you may cast a spell from among the
cards exiled with Court of Locthwain without
paying its mana cost.