D&D 5E mystic arcanum using lowlevel spell as arcanumspells?

[Kobold Stew]

Not off the top of my head. However, it's been demonstrated that I'm not the most creative "game-breaker" in the 5E community, so I've learned to be cautious.

Edited to add:

I think the real worry here isn't anything on the Warlock spell list -- I don't see anything there I wouldn't approve of. I think the question is, would a spell acquired via multiclassing, or a feat such as Magic Initiate be eligible for Mystic Arcanum? That might get weird....

Good thought - that hadn't occurred to me at all. To that (bringing in a spell from another list, regardless of reason) I would say resoundingly "no".

 

[Sword of Spirit]

Good point. In general it would be hard to break anything by using a lower level spell as a higher level one, because it's rare that scaling a spell makes it better than another spell that starts out at that level. But being able to pull in a scaled spell from another list might have an unintended consequence in some situations.

 

[JackOfAllTirades]

Good point. In general it would be hard to break anything by using a lower level spell as a higher level one, because it's rare that scaling a spell makes it better than another spell that starts out at that level. But being able to pull in a scaled spell from another list might have an unintended consequence in some situations.

Yeah, this is why I'm a big fan of ruling on a case by case basis. If a player comes up with an interesting idea that makes sense for their character and it makes the game fun, they should be rewarded for that. If they're just trying to "break" the game, and not thinking about others' enjoyment of it, bad munchkin no pizza.

Edited to add: The Mystic Arcanum class feature specifies that the spell in question has to be from the Warlock spell list. So no worries.