Addwaitya

Adwaita is one of the Geochelone Aerio species 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 the Ben 10: Ultimate Alien episode, Where the Magic Happens, and is responsible for the death of Spellbinder, Charmcaster's father, as well as Charmcaster's and her uncle Hex's villainous acts.

Adwaita is a Geochelone Aerio, from the same species as Galapagus, one of the five Andromeda aliens Aggregor was looking for: he share's Galapagus' turtle-like appearance, and was mentioned by Galapagus in Escape from Aggregor ("Thank Adwaita, it worked !") and confirmed 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
He is a turtle-like creature with pink fire-like energy surrounding his head.

Story
Adwaita's origin is unknown. At some point, he 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 reach the interdimensional world of Ledger Domain in an attempt to conquer it and all of its inhabitants. Most likely he got corrupted by the immense power and energy the artifact possesses and as he mastered magic thousands of years ago, it is likely he got corrupted over time as his race is a peaceful race. 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 youngest daughter Charmcaster to Earth in order to keep them safe. A short time after that, Adwaita had ruthlessly murdered Spellbinder 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 the Osmosian villain Aggregor reached his dimension 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. The heroes quickly had to forget Aggregor in order to get rid of the evil sorcerer, and ended up teaming up against him. After a feral fight, Adwaita was surprised and distracted by an attack from Ultimate Humongousaur, allowing Aggregor to knock him out with a sneak attack and take the Alpha Rune from him before escaping. The Ledger Domain then started falling apart, and Charmcaster, seeing Adwaita weakened, chose to stay behind to fight him and save her people.

Trivia

 * Adwaita was named after a tortoise from an Indian Zoo that lived to be 255.
 * For some reason, Adwaita does not have the physical appearance of a regular Geochelone Aerio, despite that it was confirmed he is one, thought it could be because because he is thousands of years old, or it may be the result of all the magic he controls.
 * It is confirmed by Dwayne McDuffie that Adwaita has been altered by the enormous amount of magical power he manipulates.
 * He is the first alien magician to appear in the series, and the first magician that can defeat Gwen and Charmcaster together (with Alpha Rune).
 * He is the fourth villain (Aggregor and Psyphon and Kevin being the other three), which has the power to challenge a Ultimate form but not be defeated.