A magically engineered abomination, the phasadon crawlers was created by a Mind Flayer mastermind. With a bone-like beak and tri-part snapping jaws the phasadon can cause dimensional and spacial disruptions, while skittering about on eight spidery legs avoiding hard hitting fighters and the like. Obedient and loyal like a dog the Crawlers take orders telepathically from their mind flayer masters to warp reality, pushing the limits of their enemies minds towards madness. These things can be found in the Underdark, typically with Illithid masters, however a few have been bartered (or stolen) to the Drow as well.
(For a Fortitude saving throw, simply make a saving throw and add your Con modifier)
Here is a rough sketch of what they could look like: