So, this Forgotten Beast appears in my fortress. The last line of its description was "Beware its deadly dust!" Naturally, I completely ignored this and sent my entire military to strike down the fell monstrosity. That, as it turns out, was a mistake. Everyone who came into contact with the "deadly dust" very quickly came down with advanced rot in all their skin, and turned into walking clouds of miasma. Fortunately, there do not seem to be any other effects, but it's really too early to tell whether or not my entire military will suddenly collapse with spinal rot or something.
EDIT: This just in! I looked at one of the dwarves who had been in contact with the dust, and saw on the wound screen that pretty much all her muscles are completely numb. (Also, her leg is broken, and the wound was allowed to fester, so she's not likely to live much longer.