Ritual of Roho’s Contract

At the beginning of your end step, sacrifice
Ritual of Roho's Contract unless you control a creature.
{b}: Discard a card, target opponent exiles a card
from their hand. You may cast that card for as
long as it remains exiled, and you may spend
mana as though it were mana of any type to cast
that spell. Activate this ability only any time you
could cast a sorcery and only once each turn.