The cleric in my party makes frequent use of the spell Celestial Brilliance (from Book of Exalted Deeds) and I've been doing some research on the web. Most people I've seen in other forums tend to think it's overpowered/broken. I tend to agree, but there have been various solutions suggested. Some suggested making the duration rounds/level or minutes/level instead of days/level as it is now and halving the range (60' bright light and damage to evil/120' dim light changes to 30/60). Others have suggested making it a fixed point in space (i.e. can't take it with you) and halving the range.
Short of a targeted dispel, I'm having a hard time as a DM figuring out effective and/or creative counters to it. I don't think it would be nearly as much trouble as it is except that the adventure is heavy on evil outsiders (who take double damage from it).