Old Swarm

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.

Battles with the Doctor
Swarm and the Doctor fought numerous times across time and space. (TV: The Halloween Apocalypse)

Swarm and Azure learnt the Division's greatest secret, the Mouri in the Temple of Atropos on the planet of Time that controlled time. Viewing this as herasy, the pair took control of the Temple, bringing along millions of hostages within Passengers, and evicted the Mouri. The Division dispatched the Fugitive Doctor, along with Karvanista and two other agents, to capture Swarm and Azure and restore the universe to normal. In the Siege of Atropos, the Doctor confronted Swarm and Azure in the inner sanctum and revealed she'd had the Mouri secret themselves in one of the Passengers, unleashing them to retake their positons in the Temple. The Doctor's team then immobilised Swarm and Azure and had them transmatted away. (TV: Once, Upon Time)

Swarm and Azure were officially banned from the Temple thereafter, with the Mouri being quantum-locked against them in fear of their return. (TV: War of the Sontarans)

Imprisonment
Swarm was imprisoned in a containment chamber on the Burnished Rage battleground, 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
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)