D&D 5E Question about Mental Prison

[Thehunter820]

Mental Prison

This is a 6th lvl spell from XGtE, that says it is actually to warlocks. My question is that warlocks only get 5th level spell slots, so how does that work?

 

[Umbran]

By way of the Warlock class ability: 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."

 

[Thehunter820]

Thank you I totally forgot about that ability, it had been awhile since I played warlock