Warlocks access 6th through 9th level spells through the class's Mystic Arcanum feature:
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.
Can the warlock use this feature to cast a 6th level spell as an 8th level spell?
Example: When the warlock reaches 11th level, they choose circle of death as their 6th level spell. At 15th level, they have chosen one 7th and one 8th level spell via the Mystic Arcanum feature. Circle of death increases its damage if cast using higher level spell slots.
Can the warlock cast circle of death (or other applicable spells of 7th level or lower) as an 8th level spell? Or can they only cast the spells gained via Mystic Arcanum in their original/"home" spell slots, and can only scale spells up to 5th level (the slot level they acquire "normally")?
I think the mystic arcanums are meant to be cast at the level you acquire them... if I remember correctly you don't get spell slots to cast them.
A 6th level mystic arcanum can only be cast as a 6th level spell and doesn't use up any of your warlock spell slots.
At least that's how I understand it to work.
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Correct, they aren't spell slots they are more like a single use resource similar to the once per day spells that some races get at character creation. Usable once and only at that specific level, they cannot be upcast.
Warlocks access 6th through 9th level spells through the class's Mystic Arcanum feature:
Can the warlock use this feature to cast a 6th level spell as an 8th level spell?
Example: When the warlock reaches 11th level, they choose circle of death as their 6th level spell. At 15th level, they have chosen one 7th and one 8th level spell via the Mystic Arcanum feature. Circle of death increases its damage if cast using higher level spell slots.
Can the warlock cast circle of death (or other applicable spells of 7th level or lower) as an 8th level spell? Or can they only cast the spells gained via Mystic Arcanum in their original/"home" spell slots, and can only scale spells up to 5th level (the slot level they acquire "normally")?
I think the mystic arcanums are meant to be cast at the level you acquire them... if I remember correctly you don't get spell slots to cast them.
A 6th level mystic arcanum can only be cast as a 6th level spell and doesn't use up any of your warlock spell slots.
At least that's how I understand it to work.
That's how it reads to me, too, but I wasn't sure I was interpreting it correctly.
Correct, they aren't spell slots they are more like a single use resource similar to the once per day spells that some races get at character creation. Usable once and only at that specific level, they cannot be upcast.
Think of your Mystic Arcana more like really powerful Invocations, rather than additions to your spell list.