Younger Swarm

After draining the life away from a Time Lord, the renewed Swarm revealed to the Thirteenth Doctor that he had been the Doctor's archenemy in their life as a Division agent, prior to being regressed into a child with their memories erased.

He noted that "Swarm" and "Azure" was not his and his sister's actual names but merely translations, albeit serviceable ones.

Awakening Azure
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 Passenger. 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)

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)