The passage in which the creature appears is below. I have enclosed it in spoiler tags so as not to spoil anything for those of you who haven't read the book and don't want to ruin the surprise.
There was nothing in the place except a tangled heap of old, filthy rags lying a short distance from the shaft opening and an ancient lantern a few feet away to the other side. . . .
There was a faint stirring behind him. Gord spun, catlike, his hand going instantly to his dagger. There was a little puff of dust just above the pile of rags. . . . Had some air blown just then? . . .
The heap actually had a manlike form, Gord noted as he gazed down at it—too long, too thin, but manlike nonetheless.
Then, even as he stared in horrible fascination, the rags silently twitched and twisted themselves into an even closer semblance of humanity, and from the heap an odor of mold and putrescent flesh wafted its way into Gord’s nostrils. Gord took a step back with an involuntary gasp of fear and disgust. The stuff was trying to form itself! Whatever it was, it meant them no good, and they were trapped with it! . . .
“Good work, my lady!” Gord replied with a shudder as the rag-thing flopped wetly in its efforts to raise its upper half. . .
Now the man-shaped clump of rotting fiber was in a position similar to that of a person seated on the ground, armlike appendages propping its headless torso upright, the “legs” drawing toward the body so as to enable it to arise to a fully erect stance! And a thick, wormy thing was slowly arising from within that horrible torso—a thing of sickly gray with yellow, pulsing veins visible through its membranous skin.
If this worm-creature was the “head” of the rag “body,” Gord knew what to do. Without hesitation, he stepped forward and swung the keen dagger in an arc. A moldy twist of rags flew upward, as an arm would to block a blow, but the razor-edged blade cut through the filthy cloth and struck the worm just below the bulbous upper protrusion that must have been its head.
Reeking matter splattered the nearby wall and ran down it in viscous, gray-yellow strands. The severed bulb fell noiselessly onto the rags and left a foul stain on the fabric and stone it touched as it rolled a few feet and disappeared down the shaft. A sigh seemed to issue from the heap of rags, an almost-human sound. Then the whole pile collapsed back into formlessness, making a disgusting, squishy sound as it did so.