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Can a warlock choose an upcast spell as a Mystic Arcanum?

For example, say I have a warlock and want to be able to summon bheur hags using Conjure Fey. This is slightly problematic due to bheur hags being CR 7. Could I then take an upcast Conjure Fey as my 7th-level Mystic Arcanum?

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You cannot do this

The Mystic Arcanum feature states:

Choose one Xth-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.

This requires to actually choose a spell of a certain level from your list, unfortunately the upcast version of a spell is not on your spell list and so cannot be chosen with this feature.

That said, houseruling this would likely not be game-breaking as most spells upcast to a given level are weaker than other spells of the same level.

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  • \$\begingroup\$ "Most spells upcast to a given level are weaker than other spells of the same level." Has this been conclusively demonstrated? \$\endgroup\$
    – mdrichey
    Commented Aug 25, 2019 at 21:26
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    \$\begingroup\$ @mdrichey This is true because higher level spells give you entirely knew capabilities instead of just increasing the power of previous ones. Summoning spells are one of the more edge-cases for this metric though, I'll admit \$\endgroup\$ Commented Aug 25, 2019 at 21:31
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    \$\begingroup\$ @mdrichey Another proof of this is the existence of higher level spells. If upcast spells were better then nobody would ever get higher level spells \$\endgroup\$ Commented Aug 25, 2019 at 22:52
  • \$\begingroup\$ True, damaging spells are not worth upcasting, but they are generally bad choices anyway. I would argue however, that the best 6th level spell is an upcast Banishment. Single target save-or-suck spells (Hold Person, Blindness, etc) have a good chance of failing, so completely wasting your action and your slot. If you upcast them, the chance of a complete failure drastically decreases. \$\endgroup\$
    – András
    Commented Aug 26, 2019 at 5:34
  • \$\begingroup\$ It holds in most cases. One exception is that Cleric have few good combat options among their high level spells (these do other amazing things that low level spells could never do), instead it's common to upcast Spirit Guardians and Spiritual Weapon for damage. They are rare examples of spells that scale well with levels. Other examples are Aid, the new summoning spells in Tasha's, and spells that let you choose more targets when upcast. At the very least, upcast spells are not supposed to be significantly stronger than all other spells of said level (or it wouldn't be exiting to level up). \$\endgroup\$
    – Sort Kaffe
    Commented Nov 26, 2021 at 12:09

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