Warlock: Mystic Arcanum

At 11th level, your patron bestows upon you a magical secret called an arcanum. Choose one 6th-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.   At higher levels, you gain more warlock spells of your choice that can be cast in this way: one 7th-level spell at 13th level, one 8th-level spell at 15th level, and one 9th-level spell at 17th level. You regain all uses of your Mystic Arcanum when you finish a long rest.