This cloud has the following effect:
For the player, a 1 in 2 chance for each turn in the cloud of: (1 in 3 chance of) a random mutation or (2 in 3 chance of) a bad mutation.
For monsters, if they are not magic immune, can mutate, are not res_asphyx (suggested by Eino), and pass the same random-in-HD check as mephitic cloud, will be polymorphed into another monster.
Currently, the only places it is accessible in the game is as an evaporate effect of potions of mutation (a one in two chance of mutagenic fog, or it falls through to the old random cloud; mutagenic fog is also not available as a random cloud), and as a fog generator. The effect could also be used as a monster spell, as a "trail" of cloud left behind by a monster, or possibly even for giant spore-esque monsters. As a player accessible spell, it would be a nice combination of Air and Transmutation, which I don't think exists.
As this is my first time playing with clouds, would appreciate feedback on where I've gone wrong (which is likely. :-)).