Swarm

Swarm was a dangerous being imprisoned in a containment chamber at the Burnished Rage battleground by the Division. He had the ability to burn living things to ash with a single touch, to teleport himself across space and time at will, and even 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.

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

Incarnations of Swarm
At some point, the Old Swarm and Azure attempted to harm the Mouri in the Temple of Atropos on the planet of Time. They were unsuccessful, and were officially banned from the Temple thereafter, with the Inner Temple and its Mouri being quantum-locked against them in fear of their return. (TV: War of the Sontarans)

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. During what would become En Sentac's last such tour, Swarm managed to disable all the technology keeping him frozen. Using his psychic powers to freeze En Sentac in place, Swarm kept her paralysed for a few moments before draining her life force and renewed himself into a new form. (TV: The Halloween Apocalypse)