Not-quite-undead mobs reborn from crawler-monster addicts who survive the DTs into a plodding, flesh-eating swarm.
Festering Ghouls are the end-stage of the DTs on The Iron Tangle: mobs addicted to the Rev-Up Amazing Cure-All Vitamin Immunity Shot who, denied their dose, decay through withdrawal — half die at stage one, and of the survivors half more collapse and are reborn as these ghouls. Pointedly NOT undead but a new lifeform in the old shell, they shrug off anti-undead attacks (a double-tap still helps). Level 18 and up, moss-covered and bark-skinned or worse, riddled with worm-like sores, they exist only to devour organic matter and gather with their kind, having been born knowing the route to the repair yards.
Carl finds thousands massed against the yard fence, pushing it down; among them lurk deadlier "Wrath Ghouls" that break out of their transport cages. Their danger is the swarm — and their habit of rising again minutes after a non-headshot kill, as one proves by beheading Widget mid-sentence.
Plodding, aimless mob-corpses of every original type — moss-and-bark-skinned, flaming-wolf, spider-limbed — all riddled with worm-burst sores that expose bone, moving like mindless zombies though some can still talk.
The floor's escalating swarm threat, born of stalled trains and gathering toward some ominous critical mass.
Also appears in books you've already read — spoiler-safe to their endings: