Ravager (The Halloween Apocalypse)

The Ravagers were a species that originated in 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. Azure was a gaunt shade of white, with speckles and ripples of blue, yellow and green running over the crown of her head, which was in stark contrast to her brother, Swarm. Swarm's 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 outwards, at first covering the entirety of his skull, but only around his eyes and the left side of his forehead upon changing his form.

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 disintegrate spatial objects through touch. (TV: The Halloween Apocalypse) Since they were temporal beings, they possessed a non-linear understanding of time, as demonstrated when Swarm recognised Yasmin Khan and Inston-Vee Vinder despite having never met them beforehand. He derided them for being "such linear creatures" when this surprised them.

Ravagers could teleport anywhere in spacetime within their universe. (TV: War of the Sontarans) They could use telepathy, whether casually, (TV: The Vanquishers) to form a psychic link with an individual, or even telepathically control other minds from afar, both biological like Diane (TV: The Halloween Apocalypse) or mechanical like the Doctor's TARDIS. (TV: War of the Sontarans) They could also manipulate the Time Force, which fed on spatial energy as its power source. 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".