[Taqwus]

I used Ghost Riders (and naught else) to attack an underwater C'tisian army. The enemy Marshmaster cast, among other spells, Foul Vapors.

This struck me as odd for three reasons:

(1) I used only Ghost Riders, which are longdead horsemen lead by a mounted wraith lord. All are completely immune to poison, I believed. Ghost riders never summon other units or convert other units to their side. The Marshmaster did not, IIRC, summon anything that would might be hostile to it that would also not be poison immune (Horrors would qualify, but it didn't, IIRC. Sharks perhaps as well, but I don't recall seeing any.) So why cast Foul Vapors?

(2) It's a fairly odd spell to see underwater; is it coming from vents in the sea floor, or something like that?

(3) The Marshmaster and his forces were generally not poison-immune, nor was immunity granted by any spells cast; in fact, the Foul Vapors -killed- the Marshmaster.