In Dungeons & Dragons, xorn are outsiders from the Elemental Plane of Earth. They first appeared in 1E in the original Monster Manual (1977). They are original mosnters to the game, finding not recognizable inspiration from any real-world mythology.
A xorn is described as having a barrel-shaped body with trilateral radial symmetry; three arms, legs and eyes spaced around its torso. The mouth is located at the top of the body. Because a xorn can move through earth as easily as a fish moves through water, when they choose to attack they often use ambushes, hiding just below or above a potential victim. In fact, they often move around with the lower part of their body inside the earth.
Xorn come in three categories: minor, average, and elder. They devour earthen and silicate materials, and have a taste for gems, precious metals, and magical items. Xorns sometimes attack wealthy adventurers, especially on the Prime Material Plane, where it is much harder for the xorn to find its normal sustenance. Another creature related to the xorn is the xaren. They are slightly smaller with shinier, metallic hides. While xorn feed on minerals and stone, xaren prefer enchanted metal, such as magic weapons and armor.
Xorns are very odd-looking creatures. They have radially symmetrical bodies with their large mouth at the top and surrounded by three long arms interspersed with their three eyes. Their wide, pebbly, and rocky bodies rest upon three stumpy legs. They have talons at the end of their arms. Smaller xorns are about three feet tall, but the oldest specimens could reach eight feet in height and width and weigh as much as 9,000 pounds (4080 kilograms). Xorns are immune to the effects of fire and cold and resistant to electricity.
Like many creatures from the Plane of Earth, xorns can travel through ground and stone as if it were water, leaving behind no tunnel at all. They always pass through the ground mouth-first. Xorns can sense the vibrations of other creatures in the earth and they can also smell their food from twenty feet away (six meters). A xaren could smell magical metal from twice that distance.
Xorns are scavengers, and they consume rocks, minerals, and gems. In fact, they can’t digest meat at all, so they don’t eat creatures on the Material Plane. They only attack to defend themselves, although they can be more aggressive if hungry and hunting for their inorganic food. If in groups, they would often send one of their number to negotiate for food. It is extremely likely that they would attack if their requests for food were not granted. They can speak Common and Terran.
Xorn are scavengers from the plane of Lamannia. They are devourers of earth, and enjoy the taste of exotic gems more than anything else. They are not unlike the general version of the creature, being very alien-looking creatures, with very wide bodies made of a pebbly, stone-like material. They have one large mouth on the top of its head, with powerful jaws on either side, and a row of sharpened teeth. Three long arms emerge from its torso and angle upwards. These arms end in three-fingered hands with sharpened talons. Between each of its three arms are three eyes, and at its base are three short, thick legs directly beneath one of the eyes.
In general, Xorns ignore the goings-on of beings of the Material Plane, preferring to keep their attentions, and appetites, on the earth below. Xorns enjoy eating rock, and consider the eating of rare metals a delicacy. Xorns can be aggressive when hunting for food, however, and have been known to attack people who may possess rare gems or metals.
In your campaign
If they resort to fighting, xorn often hide below the surface and erupt on their enemies, attacking with bite and talons. On the surface, they can swing their arms at all enemies around them. If they are focused on a single foe, they can rear up on two of their three feet so that they can attack with their bite in addition to their arms. If injured, a xorn would usually resort to guerrilla tactics, constantly merging into the ground for safety and then erupting again to resume attack.
You can see the xorn’s 5e stats here.