Skin Bat

A repulsive, batlike creature darts from the Stygian darkness. Its body consists entirely of rotting sheets of stolen skin. Though its large eyes are glassy and lifeless, an unmistakably evil intent glimmers within them as a toothless mouth spreads wide in hunger.
Skin bats are undead creatures created from skin flayed from the victims of sacrificial rites. They are given a measure of unlife by a vile ritual involving immersion in Abyssal flesh vats and invocations to the relevant demon lords. They feed on the skin of living beings to replenish their own constantly rotting skin. Their acidic saliva acts as a paralytic poison and leaves ugly scars on those who survive an attack.
Cliff and Dungeon Dwellers. Skin bats prey on the unwary but do not develop sinister plots of their own. Their flocks can be encountered in isolated areas accessible only by flight or by climbing treacherous cliffs. Skin bats can exist in any climate. In cool climes, they feed only infrequently, because the cold preserves their forms and reduces the need to replenish decaying flesh. This also explains why they are attracted to the dark depths of ageless dungeons. In wet, tropical climes where their skin decomposes rapidly, skin bats are voracious feeders by necessity.
Accidental Treasures. Skin bats have no use for magic items or wealth, but occasionally a ring or necklace from a past victim gets embedded in their fleshy folds, where it becomes an unintended trophy.
The typical skin bat has an 8-foot wingspan. The color of their skin matches that of their prey, so a skin bat’s coloration can change over time. A skin bat weighs about 15 lb.
Undead Nature. A skin bat doesn’t require air, food, drink, or sleep.