The food that eats you back.
Creatures of nightmare, the thankfully rare Mesnoi have unique form and attributes. Only one Mesnoi at a time will ever be "encountered".
In appearance, a Mesnoi resembles a walnut-sized chunk of freshly-roasted red meat from some uncertain yet familiar, edible animal. The insidious creature camouflages itself quite appropriately whenever it can, by slowly making its way amidst feast tables and trays of roasted meats.
Once eaten by the unsuspecting, the Mesnoi sinks down to the stomach, reforming if chewed, and begins to lap up the gastric fluids, digestive juices, and bile that it craves, like a sponge.
The Mesnoi carrier will experience mild to severe stomach pains during this time.
After a few hours of this (this is the only time that the Mesnoi can be purged with magic, or other mundane means), the Mesnoi transforms into its true form inside its victim, that of a miniature, once more walnut-sized, pot-bellied, devil-horned, snake-tailed imp. This horrid little creature then begins to chew and eat its way out of the victim from the inside out with its tiny, razor-sharp teeth, like a rat forced to do so via torture.
The victim almost always dies a slow, agonizing death. That much is certain. The devilish imp then exits its victim and begins its seventy two hour existence of mischief and malevolence, until it once more turns back into a hunk of roasted meat with the movement capabilities of a snail.