The Flux

The Flux was a cataclysmic threat described by Karvanista as a "hurricane ripping through the structure of this universe". Its impact on physical structure ran against the very laws of space-time, disrupting every particle.

According to Observation Outpost Rose's system, it had the effect of "compromising base-level integrity of all structures". This caused whole planets in its path to be completely erased. As it approached the TARDIS, the Thirteenth Doctor described this force as the end of the universe coming towards them. (TV: The Halloween Apocalypse)

Arrival of the Flux
The Flux's impending arrival rallied the Sontarans, Weeping Angels, Ravagers, and "other species from across the universe". (WC: The Flux is Coming...)

When this strange force appeared on Observation Outpost Rose, it erased the entire galaxy. While making his report of events unfolding, Inston-Vee Vinder had no choice but to attempt evacuation. His outpost station came apart to nothing mere moments later, consumed by the Flux as it advanced.

Karvanista, the Lupar bonded to Dan Lewis as his paired guardian, abducted Dan from his home in Liverpool in order to protect him from this incoming threat. The Thirteenth Doctor was first told about the Flux by Karvanista himself, when she arrived to recover Dan, ostensibly to return him to Earth. By this point, the Flux had already started. (TV: The Halloween Apocalypse) The Doctor hastily created a distress call warning that it was coming. (WC: The Flux is Coming...)

The Doctor directed the Lupari, through Karvanista, to fly into a particular formation around the Earth, together forming a protective barrier, which she hoped would keep the Flux from reaching it.

Attempting to observe this phenomenon up close, the Doctor, too, soon needed to escape, as it neared the TARDIS and turned in their direction. She unleashed vortex energy from within the TARDIS console, but this seemed to have no impact on the Flux, unmoved in its approach. (TV: The Halloween Apocalypse)

Behind the scenes
The Flux acts similar to Davros' Reality bomb. The cataclysmic event it causes is similar to DC Comics' Crisis on Infinite Earths. The Flux also appears to have many similarities to false vacuum decay, a scientific theory of an event that if happened would erase the laws of nature completely.