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 was made to destroy the universe. (TV: Once, Upon Time) 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 mysterious force appeared near Observation Outpost Rose, the entirety of the Thoribus Section was destroyed. While reporting the unfolding events, observation officer Inston-Vee Vinder had no choice but to evacuate the outpost. His station was destroyed moments later, consumed by the Flux as it advanced. (TV: The Halloween Apocalypse) The Flux ripped through Vinder's home planet, reducing it to ruins. (TV: Once, Upon Time)

The Flux caused damage to the inner sanctum of the Temple of Atropos on the planet Time, leaving two Mouri broken. This caused time to begin to run wild, beginning the Great Disruption, affecting the Doctor's TARDIS. (TV: War of the Sontarans)

A species recall was initiated, with 7 billion Lupari travelling to Earth aboard ships built to withstand the Flux to rescue humanity, who they were species bonded to. Karvanista, the Lupar bonded to Dan Lewis as his paired guardian, arrived early and 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.

Attempting to observe the phenomenon up close, the Doctor witnessed the Flux reaching the edge of Earth's solar system, only for it to turn towards her TARDIS. As she fled the wave, the Doctor directed the Lupari, through Karvanista, to fly their fleet into a particular formation around Earth, together forming a protective barrier, which she hoped would keep the Flux from reaching it. With the Flux closing in on her TARDIS, the Doctor 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) however the collision between the energy and the Flux did cause the ship and its occupants to fall through time. (TV: War of the Sontarans)

Impact of the Flux
Various factions began attempting to assert their power in the aftermath of the Flux. On her travels, Bel observed the Daleks, Cybermen and Sontarans taking control of sectors of the galaxy. (TV: Once, Upon Time) The Sontarans invaded Earth just before the Lupari shield came into effect, occupying Liverpool and preparing to embark on invasions throughout Earth's history, beginning in the Crimean War. The Doctor repelled their incursion in the Crimea and Karvanista rammed a Imperial Sontaran Time Carrier into the rest of the fleet in Liverpool, causing a temporal implosion which removed the Sontarans from Earth. (TV: War of the Sontarans)

The Doctor was able to repair the Temple of Atropos with the aid of the Mouri, however the Great Disruotion had lasted long enough to release particles of the Time Force which would erode whatever they touched. (TV: Once, Upon Time)

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 and the Nothing from The Neverending Story. 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.