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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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marked as duplicate by linksassin, V2Blast dnd-5e Aug 26 at 7:27

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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 Aug 25 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\$ – Medix2 Aug 25 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\$ – Medix2 Aug 25 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 Aug 26 at 5:34

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