It's said that their abilities include sorcery, shapeshifting and clairvoyance. They are signitories of the Unseelie Accords, and rarely deal with humans. They are led by a man named King Corb, whose court is located in Lake Michigan. They are related to Jotuns. They have been known to trade their handywork for favors and influence, and are probably the creators of the beasts used by the Denarians. They practice entropy magic, and aren't hampered by running water like most magic users. They give off a heavy anti-technology field. For some reason, the Fomor are especially prone to giving long winded, gloating, self praising monologues. 
They are weakened by iron, as they often interbred with the fae "before the Sidhe Wars."
Within hours of the fall of the Red Court , groups like the Fomor began to try to seize power and engage in genocidal tactics similar to those employed by Vittorio Malvora; only the Fomor do not kill, they twist and enslave. The Fomor do not have the resources to withstand the Wardens or remove the Brighter Future Society, hence their efforts to aid the Corpsetaker returning to human form.
As of "Cold Days" the Fomor have a strong foothold in Los Angeles, Seattle, Miami, and Boston. The White Council is fighting them mainly in Europe, and at a stalemate. In the USA the Wardens in Baltimore, San Diego, and especially Wild Bill in Texas are fighting them off. In Chicago Lara Raith and Marcone together have also pushed them out. But they are definitely gaining strength.
Their advances into the states and cities bordering large bodies of water have been the motivation for Baron John Marcone forming the Chicago Alliance between vigilantes like the Alphas, the Brighter Future Society, the outfit and local White Court nobles. 
Basis in mythologyEdit
(largely taken from http://en.wikipedia.org/wiki/Fomor)
Fomor (or "Fomorians") are a race of sea-based gods in Celtic lore. They are hypothesised to have been in Ireland before Partholon's invasion of Ireland, and engaged in long wars with the newcomers - Partholon's army, followed by his decendents the Nemedians, then the Fir Bolg, and lastly the Tuatha de Danaan. They are generally considered to represent the older native gods of Ireland, similar to the Greek Titans or Norse Jotunn. They are constantly defeated, in myth, but are never entirely vanquished; they're the equivalent of Celtic cockroaches. If cockroaches were amphibian magical badasses.
Arguably the most well-known Fomor of myth is Bres, the half-Fomor half-Tuatha who was given the role as the first High King of Ireland, in the hopes of finally uniting the two warring races. He turned out to be a tyrant who favored his Fomorian side, and was overthrown after a seven-year reign. His later defeat and death, by Lugh, forced the remaining Fomor to give up, and they allegedly return to sea.