Legend of the Caorthannach (queer-HAWN-nock)

'Neath the deep roots of the Tree of all Seasons,
Caorthannach⁠—the mother of evil⁠—resides.
Queen of the Courtless, Destroyer of Reason,
One thousand and one hundred years she abides...

Thus begins the Legend of the Caorthannach, the prophetic poem penned by the Diviner Faedor in Y669. The poem goes on to tell the story of one Telwyth-Teg, a Goblin servant and spy bound against his will to the service of Jezibaba. The jealous archfey sent her servant on a mission to sabotage Queen Mab, Archfey Regnant of the Summer Court, since few but the Courtless can cross between seasons of the Feywild.
On his way to Mab's palace, Telwyth-Teg found a hidden tunnel in the ground he assumed was a shortcut. Instead, it took him to the vast, sprawling catacombs in which Caorthannach, a demon queen who had tried to take over the Feywild during the Age of Anxiety, had been sealed away for many years. Telwyth-Teg awoke the demon, who tempted him with enormous power and convinced him to free her. He did, signing a contract with her that he would do her bidding instead in exchange for her power.
Caorthannach escaped through the tunnels, sucking dry the life-giving energy of the Tree of All Seasons on her way, and proceeded to lay waste to the city of Astrazalin. Queen Mab died in the attacks, along with countless numbers of her knights and squires. Telwyth-Teg realized he had to undo the horrors he had unleashed. He found Caorthannach's true name on the contract and used it to subdue her. He convinced her that she had signed away her powers to him rather than loaned them, and that she needed to return to her catacombs for the next 1100 years. She reluctantly obliged, and Telwyth-Teg sealed her away underneath the Tree of All Seasons once more, in a vault that would not open until that time.
Telwyth-Teg used the powers he gained from Caorthannach to become an archfey. He uplifted the Fey-orc, formed his own lordly domain to the north of the Tree of All Seasons, refused fealty to Jezibaba and instead pledged himself to the new Queen of Summer⁠—Mab's daughter Titania⁠—arguing that he saved her kingdom and thus deserved a place on the Seelie Court. He sits as an archfey of the Summer Court to this day. However, he can still travel between seasons like any member of the Courtless, thus earning him the nickname "The Solstice Man."

Historical Basis

This story would have had to take place before the beginning of the Age of Complexity, since it details the way in which Telwyth-Teg got the powers that allowed him to uplift the Fey-orc (traditionally considered the beginning of the age). However, the Age of Complexity has lasted 1111 years, with no sign of the Caorthannach's re-emergence.
When reached for comment about the timeline of their prophecy, Faedor responded: "That old thing? Darling, I'm retired. Time works strangely in the fey. Maybe she's still coming, and maybe she isn't. Could you be a dear and refill my champagne glass on your way out? Much obliged, love."