Idea I had:

At the beginning of the game, put two face down spells on the Lord Of Darkness.

When a character confronts him, he chooses another player who will take the spells and cast them at his leisure as if the LOD was a PC.

Most of the spells won't be usable, but still there will be a lot of room for surprises.

Spells that must be cast at the beginning of another player's turn cannot be cast by the LOD. The LOD won't use spells to evade the attacker, slow him down or otherwise hamper his movement. (He will mostly cast attacking spells, counterspells, or buffs/debuffs)

The prophetess can peek at the LOD's spells when she is in the Dungeon. Spells or Events that affects spell cards in hands, do affect the LOD's spells.

At the end of the fight, discard all spells that have not been cast.

At the beginning of each turn of every character currently in the Dungeon, put a face down spell on the LOD (to a maximum of two). This is the only way the LOD can draw spells.

Looking forward to your opinions.

Having non-characters use spells is always tricky, as I've do so on a couple of projects, cards, etc. Seems like you have most major issues covered, but I would suggest the spells be draw by the "player to your left" when the LoD is encountered. This means the spells won't sit there taunting the players as to which ones have been removed from the deck. In addition, it eliminates that one character (Prophetess) from needing to be an exception to everyone else in peeking at them. Personally, where an LoD or other uber-creature/enemy, she should get that exception.