Addwaitya

Adwaita (Charmcaster pronounced it add-way-cha, while he pronounced it add-wait-yah) 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.

Adwaita is a Geochelone Aerio, the same species as Galapagus: he shares Galapagus' turtle-like appearance, and was mentioned by Galapagus in Escape from Aggregor ("Thank Adwaita, it worked!") and was confirmed to be by Dwayne McDuffie. Aggregor also stated in Map of Infinity that Galapagus' species couldn't be hurt by mana, suggesting they have an affinity with it and explaining Adwaita's might.

Appearance
Adwaita is a turtle-like creature with pink fire-like energy surrounding his head.

Backstory
Adwaita, at some point, got his hands on the Alpha Rune, an exceedingly strong and powerful magical mystic item (which was actually a piece of the Map of Infinity in disguise) and reached the interdimensional world of Ledger Domain in an attempt to conquer it and all of its inhabitants. He got corrupted by the immense power and energy the artifact possesses. The inhabitants of Ledger Domain, though incredibly strong and 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 ruthlessly murdered him and took control of Ledger Domain as a ruthless dictator. He had also magically mystically enslaved most of the people of Ledger Domain to do his bidding.

Ben 10: Ultimate Alien
Adwaita was first introduced in Where the Magic Happens, when Ultimate Aggregor reached Ledger Domain 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 the Ledger Domain. When the group arrived, Adwaita immediately felt their presence and enraged by the intrusion, attacked them. Adwaitia 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 sinadome a Palorfayg attacted them and it died after falling off the edge of the land. After a feral fight, Adwaita was surprised by an attack from Ultimate Humungousaur, allowing Aggregor to knock him out with a sneak attack and escape with the Alpha Rune. Ledger Domain then started falling apart and Charmcaster, seeing Adwaita weakened, chose to stay behind to fight him and save her people.

Trivia

 * It appears that Adwaita is seemingly defenseless without his Alpha Rune (as evidence by he used it to defeat Ben and his friends but couldn't without it, he summoned his minions to fight them instead of direct assault on his part, Charmcaster was even confident that she could stop him since he doesn't have it anymore, claiming that he was weakened).
 * Adwaita was named after a tortoise from an Indian Zoo that lived to be 255 and is a Sanskrit word that means "one and only."
 * Adwaita does not have the physical appearance of a regular Geochelone Aerio, despite that it was confirmed he is one, it is confirmed by Dwayne McDuffie that Adwaita was altered by the enormous amount of magical power he manipulates.
 * Adwaita is the first non-human magician to appear in the series and the first magician that can defeat Gwen and Charmcaster together (with the Alpha Rune).
 * Adwaita is the fourth villain (Aggregor and Psyphon and Kevin being the other three), that has the power to challenge an Ultimate form but not be defeated.
 * Adwaita was a revered philosopher and mystic of his species, whom appeared to achieve godhood (but actually just transported to the dimension of Magic). No one knows that he became corrupt; as said by Dwayne McDuffie.
 * Adwaita was mentioned by Galapagus in Escape from Aggregor.
 * The fire (or something similar) surrounding Adwaita's head is similar to Heatblast.
 * Adwaita is the only known Geochelone Aerio that's "corrupted".
 * Galapagus mentioned his race is a peaceful one therefore making Adwaita similar to Vilgax in that his race was peaceful too.
 * Dwayne McDuffie confirmed that Adwaita's form is not a Geochelone Aerio's ultimate form.