Ravager (The Halloween Apocalypse)

The Ravagers were a species that heralded from the Dark Times (TV: Once, Upon Time) and worshipped the Eternal embodiment of Time. (TV: The Vanquishers) When the Division created the planet Time and the Temple of Atropos, they were able to control the flow of time through the Mouri, (TV: War of the Sontarans) which led to the imprisonment of Time's embodiment as well. The Ravager siblings, Swarm and Azure, saw this as heresy, and sought to destroy Universe One as revenge against the Division, (TV: The Vanquishers) who had imprisoned them for their part in the Siege of Atropos and the attempted freeing of Time. (TV: Once, Upon Time)

History
The Ravagers were one among several species that the Doctor warned of in a distress call following the arrival of the Flux. (TV: The Flux is Coming...)

Biology
Humanoid and crystalline in appearance, the Ravagers possessed a wide variety of skin tones and patterns. Whilst Azure was a gaunt shade of white, with speckles and ripples of blue, yellow and green running over the crown of her head, this was in stark contrast to her brother, Swarm. Unlike his sister, his skin tone was a dull grey in his original body, and a dark purple, with flecks of gold, in his new body. Crystal formations could manifest very differently as well. Azure's formations were internal, running across the linings of her face and within a crack running down the crown of her head, whereas Swarm's formations protruded outwardly, at first covering the entirety of his skull, but then later only around his eyes and the left side of his forehead upon changing his form.

Indeed, Ravagers could harvest the life force of others to initiate a regeneration into a new incarnation. During this state, their skull became encased in a layer of spikes and their skin took on a blue-red glow. This was achieved by Swarm during his escape from his containment chamber, who gained the energy required from his murder of En Sentac. (TV: The Halloween Apocalypse)

As explained by Tecteun, the Ravagers were a form of "temporal poison". (TV: Once, Upon Time) They had the ability to to disintegrate all spatial objects through touch. (TV: The Halloween Apocalypse) Ravagers could also manipulate the Time Force, which fed on spatial energy as its power source, (TV: Survivors of the Flux) teleport anywhere in spacetime within their universe, (TV: War of the Sontarans) and use telepathy, both casually (TV: The Vanquishers) and to form a psychic link with an individual. (TV: The Halloween Apocalypse) These three abilities could even be combined in the building of a psycho-temporal bridge, which allowed the user and their companions to be transported to a target with whom the user shared a psychic link, even if the target is outside of the user's own universe. (TV: Survivors of the Flux)

Behind the scenes

 * Flux was intended to featured the Masked Ravager Guards, a group of adolescent Ravagers who had to communicate telepathically because of their underdeveloped features, and were slowly shedding their skins as their crystalline rock forms emerged. This was intended to show the beginning of the life cycle that ended with Old Swarm, whose shell is cracking, deformed and crumbling. Silicone masks were made, and used in scenes filmed on the Temple of Atropos set, but these were cut from the final edit.
 * In the subtitles of TV: The Flux is Coming..., as released on the BBC America YouTube channel, the Ravagers were called "the Ravages".