Swarm was a dangerous being imprisoned in a containment chamber at the Burnished Rage battleground by the Division. He had the ability to drain the life away from a Time Lord, using their energies to fuel a regeneration of his own. After freeing himself, he 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.
Biography
Imprisoned incarnation
Imprisonment
One version of Swarm was imprisoned in a containment chamber, allegedly at "the dawn of the universe". Every thousand years, agents of the Division would come check whether he was still secure in his containment; En Sentac visited him a number of times over the millennia, though she did not know if he had literally been there since "the dawn of the universe". (TV: The Halloween Apocalypse)
Renewal
During what would become En Sentac's last such tour, performed in the company of K-Toscs, Swarm managed to disable all the technology keeping him frozen. Using his psychic powers to freeze En Sentac in place and establish a telepathic contact with the Doctor (by now in her thirteenth incarnation), Swarm kept her paralysed for a few moments before draining her life force. He renewed himself, screaming in exultation. (TV: The Halloween Apocalypse)
Renewed incarnation
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.
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)
Appearance and clothing
Imprisoned incarnation
During his imprisonment, the aged Swarm had a stout appearance. He spoke in a deep, raspy voice; his nostrils were narrow and slit-like and he had dark, squarish shadows around his sunken blue eyes. Crystal formations ringed his race, oriented towards the back of his head, giving the appearance of scales flaking off. (TV: The Halloween Apocalypse)
Renewed incarnation
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)