In a kingdom beset by a unending wizards war, one wizard sought to end it by making a warrior that devoured magic.  On the floating city of Intamon he did experiments using alchemical processes that worked too well, creating subjects that needed to devour magic at a prodigious rate or mutate into monsters.  There saliva contained the same formula that turned them into monsters and could mutate others.  There was an accident when the test subjects were being destroyed and one escaped.   It spread like a plague through the city which was evacuated and left to these abominations.  To this day no one is allowed into the city, or if entered, they are prevented from leaving on pain of death.

Spell Devourer's stand about 3 ft tall with short stumpy legs but huge muscular arms that they can use to throw themselves at sources of magic.  They are covered in the rags of there previous life but posses a small amount of natural armor due to there thick covering of hair.  They posses very little of there previous intelligence and go into a hibernation state unless they detect magic in the area, which is why the city was so easily shut down.  Once the initial mages were either turned or killed there was no magic so they simply went to sleep.

Luckily the mutations worked so that the newest infected lack the ability to create more of the formula, with only the oldest retaining the ability to make more monsters.  To this day the city is still floating, seemingly empty but full of monsters.