The Old Swarm was a dangerous being imprisoned in a containment chamber at the Burnished Rage battleground by the Division during the Dark Times. 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.
Biography
Imprisonment
At some point, 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; 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)
Appearance and clothing
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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)