In Celtic legend, there are tales of wonderment and heroics littered everywhere. This tale is of the Tuatha de Dannan. The court of the Seelie is a complexly woven arena for the competition for power among Faeries. The Seelie Court is how the Fae organize and rank themselves. The highest rank is the Queen and the position is held by Mab. Although she has been queen for all of human history, it is presumed that she became queen because she was the strongest Fae in existence. Below her is the Prime Counselor, a position of high esteem and usually a lot of power in the Fae realm.
In following with the power structure of the Seelie Court, the Fae often challenge their superiors. If a fae is victorious in the challenge, they inherit the power of their opponent, which is how Mab eventually became ruler over all the Fae. In the words of a famous wizard, you don't become queen of the Faeries by collecting bottle caps.
Faeries can and do attain a great amount and diversity of powers this way, dueling other fae and taking the spoils, as it were. Over centuries and eventually millennia, they loan favors to other faeries as well as to mortals. When it comes time to collect on a debt, they do so swiftly. As you can imagine, this can become quite cumbersome when only dealing with other faeries. In the process of dispersing and calling in markers, the Fae have found they can make the most of their advantages by involving humans. That is what happened in the Celtic tale of Sohlitaire.
In Ireland centuries past, there was an ancient fae whose true name has been lost to time, but she was called Travesty. Travesty was second in power only to Queen Mab, who ruled over the fae without mercy or weakness. That became all too dangerous when Travesty began plotting Mab's downfall. Travesty's plan was that she would trick Mab into weakening herself, and then Travesty would challenge her to a formal duel, which of course, she would win. Through that duel, she would formally have the rights to Mab's power and once won, Travesty would be the most powerful being in the universe.
Travesty had a human servant among the Fae; his name was Sohlitaire. According to the legend, Sohlitaire betrayed Travesty, double crossing her at risk to his own life. He exposed her own betrayal against Mab, and for that offense Mab banished her from the realm of the Fae, forbidding Travesty to cross over to either the fae world or the mortal one. Stuck in her own pocket dimension, she was infuriated with Sohl. As he fled to the mortal world, Travesty laid a curse upon him, binding him and the entirety of his bloodline to her. As long as she lived her half-life in her small, plain, white world, his descendants would be forced into servitude each generation.
He never found a way to lift the curse, and his line continued for millennia, all the way to this day.