Fomorians arose from the sea at the dawn of First Shem, it is said, and invaded the first nations they found. They went to war with the tuatha, the fir bolg, and the human nations they interacted with. Any nation that came near their coastal territories were attacked. Great legends have arisen about these wars, and many variations exist. But after hundreds of years of war, a mighty alliance of fey and human nations drove the fomorians out. They left on great ships and sailed away to another land, where they settled on the coasts of a massive wasteland. Legends say some fomorians intermarried with the tuatha.

Children in fomorian society are raised by enslaved people. A child is born and then given to slaves, only nursing from their actual parent for a week. Wet nurses among the enslaved then nurse them. The parents then save their milk for themselves. Once a child is about ten or so, they are brought into the house of a fomorian family--possibly blood family, possibly not--and treated like a poorly kept pet and servant. If they survive this period, they are taken on at the age of 13 or so as apprentices to the family's patriarch if they are considered male or one of the family's wives if they are considered female. If they show gender nonconformity, they are feared and sent to the priestesses.

At the age of 16 or so, they are tested. If they fail, they are enslaved. If they succeed, they are sent to another family to continue their training. When they hit their last growth spurt (usually around the age of 20), they are considered an adult and tested again. This rite ends in enslavement or independence. If they do not hit their last growth spurt, they must try to force it through ritual scarification or tear-drinking. If they are caught doing this, they are enslaved.

All fomorian society answers to a king who is determined through warcraft and treachery. The king rules a mix of autocratic and feudal society, with slaves and lords, serfs and patriarchs. Fomorian men take up to seventeen wives, though the lower their class, the fewer wives they have. Only a king has seventeen wives. Wives have roles in society, occupations relegated to them, including tending the slaves, weaving, mystic arts, healers, and overseeing the herds. Men tend to be craftsmen, overseers of the crops, administrators, builders, and hunters. The enslaved tend the crops and livestock and do all the serving, cleaning, cooking, and hard labor.

The one exception to the gender divide is the warriors. All fomorians are warriors, and the best of them are part of the warrior class that does not recognize gender. All warriors take wives (male or female). The most elite of their warriors wield slings, hurling huge stones for great distances.

Fomorians are sailors as well. They are raiders, pirates, and merchant sailors. Those fomorians who escape the class system usually join a ship to avoid enslavement. Women who become sailors must hide their gender, or at least make a show of doing so. Every ship is led by a gruama, a dark, angry warrior who is dedicated to being the first off the ship in a raid.

Fomorians are tower builders. Their estates have ceanncheathru towers along each field, paddock, fishery, etc. where the fomorian overseers watch over the enslaved workers. They have military towers in their fortifications, and towers in their castles where the king or lords live. Some fomorian cities are merely massive towers built into the sea itself. This is to avoid the massive flooding that happens along their coasts.

Because of their ferocity and hostility, fomorians are much feared by their neighboring nations. In order to keep peace, they get tribute from these nations in the form of cattle, grains, and children (who are enslaved or eaten). If they truly wish to terrorize a nation, they demand the first-born of the king or other ruler of that nation every seven years. These tributes go to the king, who may bestow some of his wealth on some of his vassals, but mostly he keeps them to pay his armies and navies. The fomorian navy is massive. Despite this, fomorians have their own form of hospitality, which they enforce bitterly. If a fomorian leader is found to have failed to meet the rules of hospitality, he and his whole house are enslaved. Neighboring nations sometimes take advantage of this.

There is a persistent legend that there is a prophecy that says the fomorians will be wiped out by one of their own, a child born into triplets. Because of this, triplets are killed at birth or turned into seals (via ithe). The legend claims the triplets will survive because one will be secreted away. Orphaned children are always killed if found.

Fomorians celebrate seven festivals each year. The biggest is the autumn festival (san fhomhar), which is a massive feast where even the enslaved get to eat. The other festivals are the anniversary of their exile (deoraiocht), the king's coronation anniversary, and four festivals based around military victories of the past. Each fomorian celebrates few feasts in their lives: adulthood, the death of their adoptive parents, and the anniversary of their last marriage.