Out of curiosity, can you think of an example where it wouldn't be a good idea to allow it?

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....

Last edited: May 25, 2015

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