Adwaita

Adwaita is a Geochelone Aerio that mastered magic thousands of years ago. He's an evil mystic being considered as the most powerful sorcerer in the universe. He was slightly mentioned and introduced in Where the Magic Happens and is responsible for the death of Spellbinder, Charmcaster's father.

Early Life
Adwaita got his hands on the Alpha Rune, an exceedingly powerful magical item (which was actually a piece of the Map of Infinity in disguise) and reached the interdimensional world of Ledgerdomain. He got corrupted by the immense magical power the artifact possesses.

The inhabitants of Ledgerdomain, though most of them were very powerful sorcerers and sorceresses, were unable to defeat him, and their leader, Spellbinder, was forced to send his brother, Hex, and his daughter, Charmcaster, to Earth in order to keep them safe. While he did that, Adwaita murdered him and took control of Ledgerdomain. He had also enslaved most of the people of Ledgerdomain to do his bidding.

Ben 10: Ultimate Alien
In Escape From Aggregor, Adwaita was briefly mentioned by Galapagus. Adwaita was first seen in Where the Magic Happens, when Ultimate Aggregor reached Ledgerdomain in order to get his hand on the Alpha Rune. Chasing him, Ben, Kevin and Gwen were forced to ask for Charmcaster's help in order to reach Ledgerdomain. When the group arrived, Adwaita immediately felt their presence and attacked them. Adwaita also used Spellbinder's voice to encourage Charmcaster to fall to her death (but the team stopped Charmcaster from doing so). When they got to Adwaita's citadel, a Palorfayg attacked them and died after falling off the edge of the land. After a feral fight, Adwaita was surprised by an attack from Ultimate Humungousaur, allowing Ultimate Aggregor to knock him out with a sneak attack and escape with the Alpha Rune. Ledgerdomain then started falling apart and Charmcaster, seeing Adwaita weakened, chose to stay behind to fight him and save her people.

It is shown in The Enemy of My Frenemy that Adwaita was so corrupt with power, he became insane and was not in the right mind in the end after his overthrow and loss of the Alpha Rune.

In The Ultimate Enemy: Part 2, Adwaita was seen in a flashback.

Ben 10: Omniverse
He has a short appearance talking to Charmcaster in Charmed, I'm Sure. He has been turned into a stone minature along with an Unknown Stone Figure and is kept in Charmcaster's bag. Charmcaster is listening to his advice in her plans but is still confident that she is in control of him.

Trivia

 * Adwaita's appearance and place of residence is reminiscent of the Marvel Comics Character Dormammu.