After draining the life away from En Sentac, the renewed "Younger |Swarm" revealed to the Thirteenth Doctor that he had been the Fugitive Doctor's archenemy in their life as a Division agent, prior to being regressed into a child with their memories erased.
After completing his regeneration, the new Swarm relished at being renewed in a healthier, more slender form. Stepping out of the containment chamber, he reduced K-Toscs to ash with a single touch and recited a mocking "Trick or treat" to the Thirteenth Doctor, announcing that "[his] time had just begun". He travelled to Earth, to a remote house where Azure, his sister, was hiding in a human guise unaware of her true nature. He killed her partner Jón and then stripped the human guise from her, awakening her true self. (TV: The Halloween Apocalypse)
A plan in motion
After the Thirteenth Doctor realised that the Flux was threatening the integrity of the universe itself, Swarm used their psychic link to have a conversation with her in a surreal dreamscape. He informed her of their past relationship, explaining that he was delighted that they could now "dance" once again. (TV: The Halloween Apocalypse)
Swarm and Azure travelled to the Temple of Atropos on the planet Time, where they were reunited with a Passenger form. They found the Doctor's companion Yaz and Inston-Vee Vinder already in the sanctum of the Mouri. Swarm used them to reveal the Mouri, who had been quantum locked against him and Azure, discovering two were broken already causing time to run wild. He destroyed two more and placed Yaz and Vinder in their places. When the Doctor and Dan arrived at the temple Swarm revealed what he'd done and prepared to expose Yaz and Vinder to the full force of time, wondering how long they'd last. (TV: War of the Sontarans) The Doctor pre-empted him by jumping onto one of the broken Mouri's positions herself, along with Dan, taking the ensuing time storm upon herself. Working with the surviving Mouri, she managed to devise a solution to reinstate all the Mouri, repairing the Temple and restoring time which Swarm congratulated her for. He revealed that, despite her efforts, time had been broken long enough to release particles of the Time Force, as planned. After revealing to Dan that they had Diane hostage within Passenger, Swarm and Azure left the Temple. (TV: Once, Upon Time)
Swarm and Azure strengthened the Time force with captive survivors of the Flux, strengthening his existing psychic connection with the Thirteenth Doctor into a psycho-temporal bridge to her current location: Division Control. There they found Tecteun and the Doctor, with Swarm killing Tecteun in revenge for his imprisonment and cornering the Doctor. (TV: Survivors of the Flux) Removing her conversion plate, the Doctor allowed two splinters of herself to return to the universe, but one remained in the outpost, where Azure revealed her and her brother's true purpose to her: to get endless revenge on the physical universe by using Tecteun's Flux to finish destroying it, then, once Time was unbound from Atropos, rewind and replay this moment over and over again for their own amusement.
Secure in the knowledge that the final Flux event was in motion, Azure and Swarm transported themselves and what they believed to be the only Doctor to the planet Time; they intended to sacrifice the Doctor, as "the first Time Lord", to the embodiment of Time, whom they viewed as their saviour. Time appeared to the Ravagers at the outskirts of the temple, appearing as a duplicate of Swarm; aware that the Flux had already failed, and still being bound to Atropos, Time chastised Swarm and Azure for failing to free them, and disintegrated them. Unlike Azure, who viewed it as an "ascension", Swarm was distressed by this turn of events in his final moments. (TV: The Vanquishers)
Appearance and clothing
After his renewal, Swarm appeared more slender and agile; his skin appeared as a greyish mauve, with pink eyes surrounded by rounder blue circles and wider nostrils that made his overall facial structure more blatantly resemble a human skull. The crystalline growths on his face were more obvious and asymmetrical, with four fang-like crystals jutting out of his right temple and a smaller cluster above his right brow and a flatter arrangement of crystals at the apex of his cranium. He towered over the Thirteenth Doctor. He wore a dark suit with prominent shoulder pads and a more colourful collar. (TV: The Halloween Apocalypse)