A tenacious undead ghoul spawned by a soul-crystal Ghoul Generator — one born for every non-undead mob that dies on the floor.
Blister Ghouls are level-20 undead ghouls unleashed by a top-of-the-line Ghoul Generator at station 72 on The Iron Tangle. Their spawn rule makes them a perpetual plague: for every non-undead mob that dies anywhere on the floor, one Blister Ghoul is born — and since every mob on the floor is dying of the DTs, the generator never stops. Unlike the Festering Ghouls, these ARE undead, and grotesquely tenacious — during the Vermillion rescue, severed heads and head-and-torso fragments blast into the cab through the windows and keep growling, clawing, and biting until Carl stomps them flat.
Their green rotting skin is pulled tight over the skull.
An undead ghoul with green, rotting skin pulled tight over its skull; so tenacious that severed heads and torsos keep moving and attacking.
A self-perpetuating undead ghoul plague spawned per mob-death, swarming the Vermillion line.
Also appears in books you've already read — spoiler-safe to their endings: