I'm in the Sewers of Arx and in my insane fight against Isbeil, I accidentally destroyed a crate of Deathfox with my Lightning Storm :\. Unfortunately, the deathfog is permanent, and is blocking the secret room I need to go into to find where the Queen is being kept. Oh shit.
Well ... apparently you can use Terrain Transmutation from the Polymorph tree (or from the legendary one-handed weapon you find in the room in the sewers) to transport the territory with the deathfog to another area so you can proceed.
Pro tip. Lol.