FireLance said:
Sorry, Abraxas, but I agree with your DM's original decision that uncovering or dropping a coin with damning darkness would not cause you to become visible. Check out the description of the invisibility spell:
To me, dropping a coin with damning darkness is along the same lines as destroying a rope bridge when your enemies are on it, or remotely triggering a trap.
The real problem seems to be that the caster created a (lasting ) spell with an AoE at a moment where no foe (to be hampered/affected by it ) was in the AoE, hence the caster staying invisible.
He THEN (actively) worked at moving said harmful effect onto the PCs/foes, affecting them. As he was not actively casting a spell, he would, by the letter of the rules stayed invisible. And I wouldn't mind ruling this way, if it had been the PCs who, of their own volition, put themselves into harm's way , like by walking into an area of Acid Fog just created in front of them as a detterent
BUT...
To my mind the caster created a temporary AoE weapon, akin to Alchemist fire through his use of "Dooming Darkness". He then used his creation to attack the PCs with it (by actively bombing/dropping itonto them ) - which would be an attack action (or just what would you label it if he had dropped a barrel-worth of Alchemist Fire onto the self-same PCs ? ), as the AoE is being harmful to the PCs (at least to his mind ). Would you honestly differentiate in such a way that a

(existing) spell AoE can be moved/dropped onto PCs without breaking invisibility, while some mundane matter might not ? Both would be an attack with a grenade type weapon to my mind.
Smells broken.
As for PC/GM access to rules. I actually don't mind (and also GM that way ) that access to magic, rules, classes and feats from certain books/publications, while permissible to the GM is limited/prohibited to players - as long as the game stays fun (hence "fair"). IMCs access to Libris Mortis, BoVD, BoED and some parts of other books are simply limited to certain groups, factions and even cultures. And unless the PCs actively seek to acquire or qualify for such knowledge ingame (and never at character generation), it might at some time become available or not.
While the GM is free to use it in the conception of his adventures, plots, and storyline. The real world is fair, evenly balanced or its possibilities clearly described, so why should a created world be ?