Okay, so I'm trying to figure out how the
glitterdust spell would interact with the miss chance for
damning darkness.
Personally, I don't think it would negate the 20% miss chance, but it would mean that Quinn wouldn't have a 50% miss chance against Tarrak due to not being able to see in the dark at all. Plus, anyone shooting at Tarrak from above (except Phar) would only have to deal with the 20% miss chance instead of a 50% miss chance. Phar, with his low-light vision, only has to deal with a 20% miss chance.
So, the spell helps those without darkvision or low-light vision see Tarrak better, but it doesn't negate the 20% miss chance due to the magic of the
damning darkness.
Does that seem reasonable?
EDIT: Found this... it is Pathfinder but it seems to tell me what should occur...
From that, I would say it wouldn't affect the 20% miss chance. It probably shouldn't affect the 50% miss chance for total darkness but I've already ruled that it does. The main point would be that if Tarrak tried to use the Hide skill, he'd suffer a -40 to the check.