Maleficent (KH III: Reconnect)

The Mistress of All Evil, Maleficent has returned from her defeat in KH 3D to finally begin her plot to conquer all worlds by capturing the 7 Princesses of Hearts once more to open Kingdom Hearts itself, since her plan stated in 3D, using the data of the worlds, turned out to be pointless and useless to her needs, according to Dr. Facilier, that is. But now, she's discovered that there are MORE princesses of Heart in the galaxy than what was originally thought to be. Her hatred and spite have given her greater control over the Heartless and Nightmares, as well as the newly arrived Heart Eater clan and returning Goblin Goons. She's even gathered new allies to replace her fallen comrades.

Eventually, Maleficent remembers something that she learned from her demonology scrolls that she read under her apprenticeship to the Horned King. Searching through the old scrolls, Maleficent discovers the prescence of a "true" Chernabog that is said to be the most powerful of all dark beings in the universe. Seeking to finally ally with the mysterious being that's responsible for "all the powers of Hell", she decides to search through her scrolls to look for an infernal spell that will allow her to summon Chernabog under her command, while also collecting the souls of her destroyed allies as possible sacrifices to the demon king's power.

This desire to become one in partnership with Chernabog slowly maddens Maleficent to the point that she is willing to recreate the Keyblade of People's Hearts from all 7 of the Princesses of Heart, and stab herself with that blade so that she can finally become darkness itself.