(From 5th Edition Monster Manual - 1994):

The most hideous and wicked of all giantkind are the godless fomorians, whose deformed bodies reflect their vile demeanors. Some have facial features randomly distributed around their misshapen, warty heads. Others have limbs of grossly different sizes and shapes, or emit terrible howls each time they draw breath through misshapen mouths. Their wretched appearance rarely evokes sympathy, however, for the fomorians brought their doom upon themselves with the evil that rules their hearts and minds.

Fey Curse. The elves remember when the fomorians were among the most handsome of races, possessed of brilliant minds and unrivaled magical ability. That physical perfection did not extend to their hearts, however, as a lust for magic and power consumed them.

The fomorians sought to conquer the Feywild and enslave its inhabitants, claiming those creatures’ magic for themselves. When the fey united to defend their realm, the fomorians fought them and were subjected to a terrible curse.

One by one, the giants fell as their bodies were warped to reflect the evil in their hearts. Stripped of their grace and magical power, the wretched horrors fled from the light, delving deep beneath the world to nurse their hatred. Cursing their fate, they have ever after plotted vengeance against the fey that wronged them.

Giants of the Underdark. The fomorians dwell in eerily beautiful caverns in the Underdark, rarely venturing to the surface. Their lairs feature abundant access to water, fish, and mushroom forests, as well as to the creatures whose slave labor keeps the fomorians fed. When those slaves can no longer toil, they are slain and devoured. Wickedness and depravity are the cornerstones of fomorian society, in which the strongest and cruelest giants rule. Fomorians mark their territories with the corpses of their enemies, painting their cavern walls with blood or stitching together limbs and body parts to make mockeries of the creatures they have killed.

Ruined Flesh, Evil Minds. The deformities visited on the fomorians prevent them from hurling rocks like their giant kin, or wearing anything more than scraps of cloth. However, the grotesque positioning of their eyes, noses, and ears gives fomorians keen perceptive abilities, making it hard to surprise or ambush them.

The greed and evil of the fomorians lies at the heart of their degeneration and fall, and continues to plague them. Fomorians make alliances with other creatures when it suits them, but they are disloyal by nature and betray their allies on a whim.

Curse of the Evil Eye. Fomorians can pass their curse onto others using a power called the evil eye — a last vestige of the giants’ once-remarkable spellcasting ability. A creature cursed by a fomorian’s evil eye is magically twisted and deformed, gaining a glimpse into the pain and malice that has consumed this evil race.

(From Bigby Presents: Glory of the Giants - 2023):

Descended from Annam’s son Karontor, fomorians once occupied a place in the giants’ ordning between hill and stone giants. In ancient times, they were scholars of magic known for their keen intellect—but also for their inflated egos and sense of entitlement. Karontor exploited these qualities, tempting them with promises of higher standing in the ordning, and incited his descendants to launch an assault on the Feywild. When the assault failed, the fomorians were banished to the Underdark and their god was consigned to a subterranean prison. Subjected to the strange magic of the Underdark, the fomorians’ bodies and souls twisted until they became the fomorians of today.

Deep Crawlers. Deep crawlers are fomorians whose bodies are adapted to crawling through the tunnels of the Underdark, perhaps in imitation or in honor of Karontor, “the king that crawls.” Their limbs are long and sinuous, and their grip is strong enough for them to climb walls and ceilings.

Fomorian Nobles. Before their banishment to the Underdark, fomorians ordered themselves based on achievements of knowledge and magical power. Before Karontor urged his descendants into an ill-fated assault against the Feywild, some of the most ambitious, inquisitive, and power-hungry fomorians felt they had already learned all they could from the Material Plane, and they departed to the Inner Planes, where they could continue their journeys of knowledge unopposed.

In their arrogance, these fomorian nobles unknowingly escaped the dreadful fate of their kin, and they remain unchanged in their remote enclaves. Occasionally they return to the Material Plane to survey the world they left. With angular features and apparently ageless faces, these fomorians resemble gigantic elves.

"The fomorian nobles I have met are so utterly unrepentant and so unmoved by the plight of their cursed kindred that I struggle to embrace them as my cousins."

—Diancastra

Warlocks of the Dark. Some fomorians search for the arcane glory their kind possessed before their banishment into the Underdark. They forge magical pacts with entities of the dark—perhaps powerful Fey of the Gloaming Court or eldritch entities buried deep in the Underdark. Their pacts give these fomorians power over shadows, allowing them to mold and shape darkness like clay.

(From 2nd Edition Monstrous Manual - 1993):

Fomorians are the most hideous, deformed, and wicked of all giant-kin.

The fomorian giants are all grossly deformed behemoths. Each has a different set of deformities, which must be determined by the DM. A partial list of deformations includes misplaced limb, misshapen limb, misplaced facial feature, hunchbacked, bulging body part, drooping flesh, body part too big or too small, flapping ears, huge snout, large feet on short legs. Their thick, hairy hides, combined with the pelts and odd metal bits they wear for protection, give an effective AC of 3. They have scattered patches of hair as tough as wire on their pale white skin. Large warts and other growths are scattered across their bodies. There is no single odor associated with fomorians; some smell strongly due to overactive sweat glands, others have no smell. Their voices are also each different due to their unique deformities.

Combat: Fomorians use all manner of clubs and other blunt instruments. Regardless of the weapon, it inflicts double damage plus 8 points for Strength, while their fists alone inflict 2d4+8 points of damage. Their deformities prevent them from hurling boulders as true giants. They work any bits of metal they can find and scavenge into their clothing, to aid their Armor Class. The typical fomorian is AC 3, while a particularly well-armored one, or one with a shield, might get an AC as good as 1, but no better.

Typical fomorian strategy is too sneak up on an opponent and hit him as hard as it can. It works well for them since their opponents suffer a -2 penalty to their surprise rolls, because the fomorians move slowly and carefully. These giant-kin are only surprised on a 1 on the 1d10 surprise roll, because they tend to have eyes and ears in odd places on their heads. If the fomorian bothers to keep an opponent alive, he is crudely tortured until dead, and then eaten.

Habitat/Society: Fomorians live in mountain caves, abandoned mines, or other subterranean realms. They rarely modify their homes, but adapt to what is already there. These deformed giants wander throughout the underground complex, for almost any distance, stopped only by hazards they do not want to challenge. A fomorian clan picks a small, (to them) defensible alcove for a lair. Their territories are sometimes marked by the bodies of their enemies. Their treasure consists only of stolen items from enemies. Pieces of armor are added to their own patchwork protection. Since they do not care for it, this armor quickly deteriorates and becomes worthless.

Their society is ruled by depravity and wickedness. The strongest and cruelest giant rules over all the others within reach, which is usually a small number. The women and children are treated as slaves. Acts of violence are common among fomorians, sometimes resulting in permanent injury or death.

Fomorian giants have been known to work with other creatures for evil causes. Usually the other creatures must completely dominate the fomorians, or be capable of it, to form the alliance. Such an agreement lasts only as long as the fomorians fear their cohorts. Once their interests no longer coincide or the fomorians no longer feel threatened, they double-cross their partners, as quickly as possible.

Ecology: These twisted giants can live for weeks on little or no food. This is good, because their underground dwellings do not provide an abundance of it. They can eat almost any organic material, including fungi, lichens, plants of all sorts, bats, mice and fish. They particularly savor the taste of large mammals, especially those that beg not to be eaten. Preparing a meal usually involves torture rather than any efforts to improve its taste.