Seelie

The word "Seelie" comes from the Scottish lowlands, describing fae that are "happy, blessed, lucky, kind, and bringers of good fortune". The Seelie Court is described as court of pleasant, happy people and the members of it are known to be benevolent for no other reason than the sake of kindness. They are extremely generous to those that that favor and often give gifts of corn and bread to the hungry and poor. They are also known to do things around the house for humans, such as in the case of the brownie.

But their kindness should not be confused with ignorance and it should not be assumed that they're easy to use. Members of the Seelie Court will still kidnap humans and infants (replacing them with their own as seen with changelings). They will readily avenge any wrongs or slights against them and can be dangerous when offended or harmed. Seelie fae will generally warn people at least once before retaliating.

Fairy Rade

rides out as they please and is described as a grand procession of Fairy knights in an armed excursion. Fairy Rades are dangerous to any humans they come across as well as any unaligned fae, but also offer the opportunity to free captives to those brave enough to confront them. They generally hunt down the Slua Si.

Unseelie

The word "Unseelie" means "dangerous, unlucky, unfavorable, unhappy, ungodly". The Unseelie Court is described as always unfavorable to human and is closely linked to the Slua Si, the malicious Fairy Host who torment people and cause illness and death wherever they go. They are known for kidnapping mortals and using them for entertainment before dropping them back far from where they were taken.

Just as the Seelie Court could cause harm if motivated, so too the Unseelie denizens may act kindly toward humans.

A note on the Unseelie: Although it's trendy now to see the darker fae as just as kind and helpful as the Seelie Court, merely grumpy and misunderstood, in folklore there was usually a good reason people feared them and that reason was their tendency towards homicidal reactions and eating people. Contrary to what most young adult novels and paranormal romance currently on the market like to say, the Unseelie Court aren't the emo bad boys of the fae world who just need a big hug and some understanding. While it's true that it is not a cut and dry situation, we need to be careful not to romanticize the more dangerous fae based on our own desire for them to secretly be kind and gentle (Fairies: A Guide to Celtic Fair Folk, Morgan Daimler; 2017)

Slau Si

Group of malicious Unseelie fae that is a combination of fallen Aos Si, other unseelie and the living undead. They hunt in a raid at night and attack humans that get in their way along with any solitary, unaligned fae who have either never been aligned, or have fallen out of favor with either the Seelie or Unseelie court. They are hunted down by the Fairy Rade.

The Courts in General.

Both courts are ruled by a species called the Tuatha De Danann, who eventually became known as the Aos Si and are comparable to elves (see species for a more detailed description).

The courts are comparable to the monarchies of the mortal world but rule based on season. The Seelie courts rule over the seasons of Spring and Summer, while the Unseelie Court rules over the Autumn and Winter.

At the head of the court is a King or Queen, whose lineage are descended from Nuada, leader of a previous wave of inhabitants of Ireland. They came from four cities to the north of Ireland—Falias (Spring), Gorias (Summer), Murias (Autumn) and Finias (Winter) - where they taught their skills in the sciences, including architecture, the arts, and magic, including necromancy (primarily Unseelie).

Though the Monarch is the supreme ruler of their designated court, they are influenced and advised by an elected council. The members of the court who reside within the ancient citadels are pureblooded Sidhe, who are becoming rarer with each generation.

During the season in which a court is in power, festivities and feasts are held within the respective kingdom. Over all decisions concerning the fae are made by whichever court is currently in control.

Seelie Court

Known as the Light Court, are the fae that are more inclined towards kindness to humans and are more likely by nature to have interests or goals aligned with humans. The magic of the light tends towards healing, blessings and increase. They rule over the seasons of Spring and Summer, bringing bounty, blessings, food and fertility to human kind. The magic of the Aos Si of this court tend to be of the elements of earth and water.

Unseelie Court

Known as Dark Court, they lean towards seeing people as a resource whether it be for food or entertainment. The Unseelie Court is comprised primarily of solitary fae of a malicious nature who feed on or enjoy hurting mortals. However, they do still have a system similar to the Seelie consisting of the Sidhe who rule over a more structured monarchy. The magic of the Unseelie court tends towards decay, ill-luck, and transformation. The magic of the Aos Si of this court tend to be of the elements of fire and air.