In the decrepit village when I fought the Duergar they were able to successfully use their spells while under the influence of the sussor bloom.
Before the encounter I cleaned them out of all their magic scrolls and the moment the fight began with Dekh I was planning to just shove him off the side but he cast invisibility on himself.
I suspect maybe it's like a channel divinity spell because no other spells were cast during the battle.
