Ok, so sadly I guess this game goes back into hibernation for a few more years. Over many hours I progressed through the mazes of the Castle of Mirrors and reached level 3 and there encountered a number of monsters named 'Shimmering Wall'. This monster applies the same effect as the trap - applies the effect that almost always summons demons. It does it on EVERY hit. It also looks like a normal wall, so it is very easy to stumble right into it. Yay.
So I tried to cast my ace-in-the-hole banish demons spell and it won't let me - 'Ah, but the colours are too beautiful for that'. I lasted 5 rounds by using a gleaming wand, but nope, being immediately surrounded by endgame monsters is too much even for my L19 Chaos Warrior Mage. I honestly see no way to survive this attack/trap once it fires. You can't outfight it, you can't run, you can't use any magic to help yourself, you can't resist it, you can't heal your way through it, you can't dispel it, and you cannot hide/survive till it wears off. So *Boom* guaranteed death.
If anyone ever figures out how to handle this trap/effect I'd love to know how!