Sentris

Sentris was the main Neverperson in the Antiverse. She arranged an elaborate plan to destroy the Eye of Harmony, based around the legend of Zagreus.

History
Sentris was the 422nd Coordinator of the Celestial Intervention Agency on Gallifrey. She got curious about the use of the Oubliette of Eternity: a Time Lord execution device that would "disperse" people out of the Web of Time. Such people would have never existed, and the Web of Time would adjust for their absence. Sentris checked the records in the CIA's time-proof vault and was horrified to discover she had personally consigned over 200 people to dispersal in the previous year alone. Consumed with guilt, Sentris dispersed herself in the Oubliette.

However, instead of canceling out her existence, the Oubliette forced her into the Antiverse. This realm was composed of Anti-Time, and contained the souls of the thousands of people dispersed in the Oubliette over Time Lord history, including many sent there by Sentris herself.

Sentris was outraged by the circumstance, and vowed to destroy the universe of Positive Time. She crafted a careful trap in the guise of an ancient TARDIS stranded in the Antiverse. She and other Neverpeople then used what little contact they had with the normal universe to seed legends of Rassilon and his quest to destroy Zagreus across history. When a prime breach developed between the two universes in the form of Charlotte Pollard, Sentris took her form to honor her.

The Eighth Doctor's TARDIS and Time Station came through the breach, carrying Lady President Romana and CIA Coordinator Vansell. Sentris convinced them that Rassilon's remains were sealed in a casket, and handed it off for transit home to Gallifrey. In reality the casket contained a large store of Anti-Time, ready to wreak havoc on the normal universe and the Eye of Harmony. Exposure to Anti-Time infected most of the Time Lords, and Sentris took control of the Time Station.

Sentris piloted the Time Station back through the breach to the normal universe, and set the Time Station to self-destruct once it reached Gallifreyan space. But the Doctor materialized his TARDIS around the Time Station, containing the resulting explosion. Sentris was annihilated in the process.

The Anti-Time energies and the combined hatred of Sentris and the other Neverpeople were absorbed by the Doctor and his TARDIS. The energies overwhelmed the Doctor's normal personality and he became a champion of chaos: Zagreus.