Mystic Arcanum (9th Level) - Wish

At 17th level, your patron bestows upon you a magical secret called an arcanum. Choose one 9th-level spell from the warlock spell list as this arcanum.

You can cast your arcanum spell once without expending a spell slot. You must finish a long rest before you can do so again.

You regain all uses of your Mystic Arcanum when you finish a long rest.