Fomorians are a species of giant originating from Irish mythology, characterized by their hideously deformed appearance and proclivities for dark sorcery.
In Dungeons & Dragons, traditionally, the fomorian is essentially a grotesquely ugly and evil giant that ranks even lower than the Hill Giant on the Ordning. Deformed and malevolent, they are so physically warped they can't even hurl boulders like every single other species of giant in D&D can. They are cavern dwellers whose crude tribes consist of one alpha fomorian and whatever other unfortunates he or she can bully and torment, spending their time looking for victims to torture and eat.
In 4th edition, they changed; the 4e fomorian is a fey giant, believed to be an echo of the Feywild, whose great empire once warred with that of the Eladrin. Ultimately defeated, they were exposed to magical backlashes that devastated their bodies and minds, leaving the once-beautiful giants as hideously deformed and insane monsters. 4e Fomorians are best defined by their "Evil Eyes", a large and distorted eye that serves as a focus for magical energies, allowing them to inflict all kinds of deadly magical attacks on their enemies. Too insane, paranoid and malevolent to form a cohesive threat, their petty empires still throng the Underdark of the Feywild, backed by loyal cyclops servitors and reigning over armies of slaves, such as the gnomes.
The 5e version of the fomorian is basically a watered-down version of the 4e version.
The frustratingly hard to pronounce (unless you're Welsh) Fhoimorien is a giant species native to the setting of Birthright. These deformed giants are the Cerilian equivalent of Fomorians, and are malicious, mystical creatures with an innate elemental affinity for earth. This manifests in their innate ability to cast Stone Speak/Tell, Animate Stone and Passwall once per day, but many of them become "warlocks", which are 1st to 4th level Wizards whose spellbooks are filled with earth elementalism spells. Weirdly, fhiomoriens can assume gaseous form as a cloud of pale blackish-green, foul-smelling smoke twice per day, and are adept at using this to escape from threats, manuever around battlefields, or otherwise cross normally impassible terrain. Perhaps because of this ability, fhiomorien warlocks also favor air elementalism spells alongside their earth elemental arsenal.
Fhoimoriens inhabit desolate marshes and forests from the seashores to the mountains, as well as live in deep caves.