1x Found spell 'Searing Brand' using id 213148 on 15, tell the authors!
BigWigs_Core\BossPrototype.lua:1141: in function UnitDebuff' BigWigs_Legion\Nighthold\Aluriel.lua:417: in function ?'
BigWigs_Core\BossPrototype.lua:456: in function <BigWigs_Core\BossPrototype.lua:422>

Locals:
InCombatSkipped

Does this happen on difficulties other than Heroic?

Just tested it again; looks like it only happens on Normal difficulty. No issues on Heroic or Mythic.

Ah, makes sense. Yes, I didn't log out between difficulty checks. Thanks for fixing!

You probably killed her on the other difficulties before she had a chance to apply the debuff. That error message in your initial report says "on 15", 15 is the ID of Heroic difficulty.

Anyway, was the ID you got on Normal 213148 as well?

Yes:

1x Found spell 'Searing Brand' using id 213148 on 14, tell the authors!
[string "@BigWigs_Core\BossPrototype.lua"]:1168: in function UnitDebuff' [string "@BigWigs_Legion\Nighthold\Aluriel.lua"]:417: in function ?'
[string "@BigWigs_Core\BossPrototype.lua"]:474: in function <BigWigs_Core\BossPrototype.lua:440>

Locals:
Skipped (In Encounter)

And on all difficulties, I waited the entire time until she cast Searing Brand on me before I killed her, so I don't know why the Heroic/Mythic behavior would be different, unless something has changed behind the scenes since I opened this ticket originally.

Thanks for checking.

I looked into it, the error would occur if you had both https://www.wowhead.com/spell=213148/searing-brand and https://www.wowhead.com/spell=213166/searing-brand at the same time. Also BigWigs remembers spells it complained about in the current session, so if you tried all difficulties without logging out, that could be another reason you only got 1 error.