Vitalis, et al. get outta here!

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So one of my players wants to take Hellknight Dedication, and I, as the GM, wanted to actually do the Hellknight Test (single combat to the death against a devil overseen by a Hellknight) as a proper story event rather than just handwaving it. I have basically everything planned out, except that I have zero clue how to make it engaging for the rest of the party.

I definitely can't have them use, like, damaging spells or whatever, because that defeats the purpose of it being single combat. I'm just trying to think of a way for the rest of the party to feel involved without detracting from it. I'm definitely putting a hard line of "the rest of the party can't cast spells while the Test is going," but other than that I'm not sure.