Tecteun (Once, Upon Time)

An incarnation of Tecteun survived the destruction of Gallifrey by, having been on the Division's space station in the Void between universes at the time. (TV: Survivors of the Flux) She encountered the Thirteenth Doctor twice, once right after the Doctor left her friends' time streams, (TV: Once, Upon Time) and again aboard Division Control. Responsible for the Flux and the release of Swarm, she met her demise when the Ravager disintegrated her in revenge for her role in his imprisonment. (TV: Survivors of the Flux)

Biography
This incarnation of Tecteun survived the destruction of Gallifrey by, thus becoming the de facto leader of the Division. Tecteun was aware that the Thirteenth Doctor, a late incarnation of her adopted child, had discovered the truth of the Timeless Child and knew that the Division had erased her memories of her original life. The Division, under Tecteun, who knew that once the Doctor knew the truth she'd never stop "hounding" them, manufactured the Flux to destroy the universe, which they would then use the energy from to propel their space station into the next universe, which they called Universe Two. (TV: Survivors of the Flux)

While the Flux destroyed space, however, Tecteun suspected that it may not be enough. She considered the Ravagers, Swarm and Azure, to be "rare and useful creatures" that could act to disrupt the flow of time as a "temporal poison". (TV: Once, Upon Time) Indeed, Swarm knew that he had been released from his containment chamber by Tecteun. (TV: Survivors of the Flux)

When the Doctor entered a time storm in an attempt to repair the Temple of Atropos and reinstate the Mouri, Tecteun, while keeping her identity unknown, informed the Doctor that she was fighting a lost cause, that the universe was over, and that it was all the Doctor's fault. With that, she advised the Doctor not to come looking for her and told her she could go. The Doctor reawoke in the repaired Temple, but discovered that she had been too late as the Ravagers had already harnessed particles of the Time Force. (TV: Once, Upon Time)

An Extraction Squad of Weeping Angels were later sent by the Division to retrieve a rogue Angel that had knowledge of all Division's history. The rogue Angel, knowing that the only thing the Division wanted more than it was the Doctor, made a deal that would spare it in return for the Doctor. (TV: Village of the Angels) Despite this, the Extraction Squad went back on their word and abducted the Angel anyway.

Upon being transported to the Division's outpost, she introduced herself as Tecteun to the Doctor. Tecteun explained that because of the Doctor's actions in bringing hope to the universe, she had always been inadvertently interfering with the Division's plans, although Tecteun had thought she was "manageable" until she discovered the truth about the Division.

Offering to restore the Doctor's memories and take her to the new universe, Tecteun's confrontation with the Doctor came to an abrupt halt when Swarm and Azure used a psycho-temporal bridge to cross into the outpost. Insisting that they didn't belong there, Tecteun stood up to Swarm. In return for Tecteun releasing him, Swarm disintegrated her, thus leaving the Division in a power vacuum. (TV: Survivors of the Flux)

Behind the scenes

 * The name "Awsok" was not used by any characters, neither in Once, Upon Time or Survivors of the Flux. Instead, the alias originated from the credits of Once, Upon Time, to conceal her true identity.
 * Chris Chibnall implies during The Flux: Story Breakdown that Tecteun was aware of revealing the secrets of the Timeless Child and the Division to the Thirteenth Doctor during The Timeless Children, and that she unleashed the Flux to counter the Doctor now that she was actively searching for the Division. He also elaborated on her apathy towards others, explaining that she ultimately saw everything as an experiment, describing her as "the ultimate scientist with no regard for the moral context of what she [was] doing".