This creature appears to be a grayish-brown rat about a foot long.
Soul Nibbler CR 2
Soul nibblers are immune to all spells of the Necromancy school.
Essence ingots are small rectangular bricks of blackened stone that contain the soul of a living creature. Creatures are trapped in essence ingots through the use of a great smelter, the location of which has been lost.
A trapped spirit can be released by casting remove curse (or similar and more powerful magic). A released soul reforms its material body in 1d4 rounds. An essence ingot has a hardness 5, hp 10, and Break DC 20. Note, however, that if an essence ingot is broken, the trapped soul is not released but is lost forever.
Organization swarm (10-20)
Soul nibblers are rats that have undergone genetic mutation due to continued feeding on essence ingots (see sidebar). The biological effects of a prolonged consumption of soul stuff are minimal. Soul nibblers are always black in color, and their eyes sometime glow with a sickly light. The magical changes wrought by their unholy diet, however, are more profound, granting the soul nibblers unique necromantic abilities.
Having begun life as normal rats, soul nibblers behave in combat much like their mundane cousins. The primary difference between soul nibblers and standard rats is their bond to dark magic, which grants them the powers of energy drain and necromantic immunity.
Soul Nibbler from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Authors Travis Hawvermale and Scott Greene.