The Fomori are the creatures which began emerging from the Gate of Horn after it had been damaged by Lord Dougal. After the Sundering when they first appeared, they were few in number. However, they grew in numbers, more and more coming through the Gate that eventually they were strong enough to expel the Sidhe and other fae from Arcadia. During this time no Sidhe came through the Gate, no matter how many were killed.
At the time of the Shattering the Sidhe host marched on Arcadia, intent on destroying the Fomori. Whether they succeeded or not is unknown.
The Fomori are terrible, twisted creatures, sharing none of the awesome grandeur and beauty of the Sidhe, though they exert a fascination on those who look upon them. They have considerable power over the Dreaming. When House Scathach used their magic to open paths directly from mortal world to Arcadia, the Fomori used their own powers to destroy them - in the end the paths shattered, though not until the Sidhe host had long since begun their march.