If that wasn't literally one of the mist/fog covered rooms (there are precisely two that have naturally occurring normal fog -- elemental rooms and th'actual mist-filled ones -- and then one with acid mist and another with nausea inducing gas), then it wasn't a surprise
And if the fog was moving at all, it definitely wasn't.
Whenever you see fog, anywhere else, that means you're run into one of a very specific set of critters. Of them, something like two (One or two of the nymphs, and fog elementals. Maybe something else I'm forgetting.)
aren't primary spellcasters of some sort. All the rest of them (note: This includes higher level dragons) are going to be spellcasters. So if you see a moving fog bank -- you know that's almost certainly a caster. Pray it's using a druid spell list and not a wizard's.
Really need to check the repository's issue list thingy, but speaking of that, has anyone mentioned the whole "Spellcasters generate with every bloody sustained spell they have running" thing? Higher CL wizards are the
worst thing. Not necessarily because of their vicious spells or anything, but because they usually generate with somewhere in the range of multiple thousand mana worth of sustained spells running. And if you're not a caster (or willing to use wands, but good luck with
that when you've ran into something running a wandweird spell that has a radius larger than your screen.),
doing anything to something like that is... not very viable. You hit them, and take 30+ damage from stacked fire/death/etc. shields. You fire projectiles and hey, missile protection. You try to go reach (which
usually ignores melee-ranged retaliation) and... hey, they're larger than normal and punch you in the face anyway. Also they've
probably been blessed with a few hundred more HP than they have any right to have, ha.
Bloody nuisances. Usually really easy to
avoid because having all those sustains up means they're a gigantic multicolored (from sphere of invuln/wandweird/etc.) rolling fog bank (obscurement), but still ruddy annoying.