Nafaria's Star Challenge

When Nafaria's Star Challenge 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 cards exiled with
Nafaria's Star Challenge without paying its mana
cost.