Glitterworm

Kin to the purple worm but only 5 to 6 feet thick, the glitterworm is a long, leech-mouthed burrower that can shift its skin to any earthy or metallic hue; slate gray may be its natural color, though nobody is sure. Rare among surface-world predators, it feeds on creatures of elemental earth.

It lures prey by taking on the sheen of platinum, gold, silver or another precious metal, and it can mimic the smell as well, even of rarities like mithral and adamantite. A victim sees a thin vein of priceless ore and hurries over to dig or eat. The worm then sprays a straight jet of acid 70 feet long and 5 feet wide for 3d10 damage, halved by a save vs. breath weapon, and lunges to fasten its dripping jaws, inflicting 2d10 each round it stays latched. The acid keeps flowing, so worn and carried items must also save. Breaking free takes a Strength roll at -1. Neither acid nor any earth-based attack can harm it.

Glitterworms live alone and meet only to mate, coiling together for an hour before parting. The female lays one glossy gray egg about 3 feet across that no surface creature can eat, though elemental earth beings can, one more reason the worm hunts them down. The egg needs no tending and gets none.

It eats anything: flesh, plants and stone, and its taste for elemental earth likely grew out of its habit of eating ordinary rock. Xorn are a favorite, worth abandoning other prey for, and dwarves, gnomes and other subterranean delvers are close behind. Metabolic changes over its evolution leave it without natural foes anywhere but the Elemental Plane of Earth, where it is still more hunter than hunted.

Adventurers and wizards keep trying to kill them or steal their eggs to raise as guardians. Since those jaws can chew through anything not magically protected, no ordinary enclosure holds one, yet reckless wizards still pay 5,000 gp for an egg, and alchemists pay up to 100 gp per vial of acid.