Tatiana

Queen Tatiana Medb is the sole ruler of the Fey Wild. She is supremely beautiful, which is paled only by her cleverness and her cruelty. Unlike Oberon or Harlequin, she embodies the Seelie and Unseelie halves of the Fey Wild entirely, accepting both sides of mortal consciousness into herself. She is loved and feared, wonderful and terrible, the True Queen of the Fey.
Her origins are mysterious. She came into existence as a Greater Fey on the border of the Seelie and Unseelie territories. Some think she was created by Oberon as the rest of the Fey Wild was, but she believes she is an existence of her own. Known as the "Twilight Lady", she ruled over denizens from both sides. Eventually, Oberon took notice and, struck by her beauty, decided to make her is wife and, when she refused, defeated her in combat and dragged her to his court by force. Eventually, she submitted to him and became his loving bride, or so it seemed...
She is the Mother of Eldonna, Llothellenell, and Girallon, who would all eventually ascend to godhood. She was extremely angry when Llothellenell was torn from her arms as an infant, and again when Girallon was plunged into Hell by Oberon for helping his sister steal divinity. At the time, she was powerless to stop Oberon, but did refuse to birth any more children for him (though he was also done with children after Eldonna's betrayal).
At some point, Oberon's cheating with a nymph, calling her the most beautiful, was the final straw, transforming Tatiana into the creature, Lady Marrowick, out of jealousy and rage. She cursed Oberon to forget about her, and corrupted a portion of the Fey Wild. She was eventually transformed back by Sathias's encouragement, and by that point, had stored enough power to challenge Oberon. Besting him in a fight, she managed to take the throne from him, and the majority of his power, just enough to keep him placated and subservient for the time being. But to most it seemed Oberon was merely being punished for a while in a marital spout.
In truth, she began conspiring with Harlequin, giving him the core of the Winter Queen so he could further challenge Oberon, and telling him about the Grails, which she learned by spying on Raina through a lock of her hair. She did all of this to cause the final confrontation between Harlequin, Raina, Opal, and Ira. Which she'd hoped would lead to Harlequin's death, and thus, Oberon's. Her plan succeeded, and she took both Harlequin and Oberon's position as the sole ruler of the Fey.

Post-War:

What she didn't account for, however, was the petty vengeance of a certain three heroes, who may have picked up a special gift along the way from a certain god of chaos. Ira, Opal, and Raina hatched a scheme to get Tatiana back for plotting against them, sending Opal into the Fey Wild with the "Terrasque in a Box", where she, after warning her mother and the other seasons what was about to happen, shattered it open near Tatiana herself. Then, Ira popped her back out of the Fey Wild and into Arborea, where they, and Tatiana's children, watched with delight as Tatiana was chased down by the giant spiky magic-immune monster, and swallowed whole. The beast went to sleep soon after, having eaten its fill of magic energy, and the destruction to the Fey Wild was quickly mended. Some say that Tatiana lives on in the beast's gullet, awaiting her escape, but has made no sign of reappearing in the centuries since, allowing the Seasons to rule the Fey Wild as a group.