[semlar]

I would be very surprised if UNIT_SPELLCAST_SUCCEEDED didn't fire for this spell.

Try using RegisterEvent instead of RegisterUnitEvent, and then filter by source after you determine if it fires.

RegisterUnitEvent sometimes doesn't behave the way you would expect it to.

Also don't assume you have the spell IDs correct, watch for the spell name if you need to filter it or just don't filter it at all if it isn't too spammy, then once you know you have the right spell ID you can watch for that.

Occasionally scripted spells aren't considered "cast" by the player and can't be tracked directly, but they're relatively uncommon.