Fiend Mutton is a type of food found dropped by beasts in the Cathedral of Syndetic Ascension, on levels 2 and 3. Problem Sleuth was able to obtain two of them in level 2 and ate them to completely restore his missing Vitality Carats. The Hired Muscle was also able to obtain some pieces of Fiend Mutton of his own before Problem Sleuth, somewhat irking him in the process.
The item is able to restore at least 4 Vitality Carats, seeing as Problem Sleuth had 8 Carats missing before eating the two Muttons.