Can a warlock choose a lower-level spell upcast at a higher level for their Mystic Arcanum class feature?

Question

A player asked me if they could use their 17th-level Mystic Arcanum (Warlock class feature) to select the 6th-level spell "Mass Suggestion" cast at 9th level (to always make the suggestion last a year and a day).

I am unsure how to answer this (final verdict), as Mystic Arcanum specifically states:

At 11th level [...]. Choose one 6th level spell from the warlock spell list as this arcanum. [...] At higher levels, you gain more warlock spells of your choice that can be cast this way: [...] and one 9th-level spell at 17th level. (PHB p.108)

So my take on this is that they are very specific about which spells you are able to choose (spells that are on the warlock spell list, and there, they have to be n-th level).

Am I wrong? Is there any rules citations which support what the player asks?

Answer 1

Spells don't take on a higher level until you cast them using a higher level spell slot.

From page 201 of the PHB, under "Casting a Spell at a Higher Level" (emphasis mine):

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting.

You are not casting the spell when you pick it for your Arcanum so no, you wouldn't be able to select Mass Suggestion as a 9th-level spell when you reach 17th level.

Rules designer Jeremy Crawford has also unofficially confirmed on Twitter that unless the DM allows it, you can't choose higher-level versions of lower-level spells for Mystic Arcanum (e.g. choosing a 9th-level version of a 6th-level spell at warlock level 17).

Brendan Smith - @spaceysnorlax
@JeremyECrawford Can warlocks learn lower level spells as mystic arcana (eg circle of death as 7th or even dominate person as 6th)?

Jeremy Crawford - @JeremyECrawford
@spaceysnorlax As written, Mystic Arcanum doesn't allow spells of lower levels, but a DM could certainly allow it.

Answer 2

Warlocks only have one spell level for all their spell slots (and they are very few in comparison.) the highest spell slot level they achieve is 5th. This deficiency is augmented by eldritch evocations, which allow many lower level spells to be cast at will, essentially turning 1st and 2nd level spells into cantrips, and mystic arcanum which are limited to being cast only at their spell level - unless the dm chooses to manipulate these restrictions.

Answer 3

Answered in discussion offsite, but I'll copy it here in case it's helpful to anyone else:

From the Player's Basic Rules:

CASTING A SPELL AT A HIGHER LEVEL

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level. Effectively, the spell expands to fill the slot it is put into.

Some spells, such as magic missile and cure wounds, have more powerful effects when cast at a higher level, as detailed in a spell’s description.

Thus, Mass Suggestion cast in a 9th level slot is a 9th level spell, and it is on the Warlock spell list.