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) The Flux's main component was anti-matter originating from outside the universe. (TV: The Vanquishers)

First Flux event
The Flux was created to destroy the universe, (TV: Once, Upon Time) using anti-matter, (TV: The Vanquishers) by the Division, who under the leadership of Tecteun had decided to abandon the universe for the next one. (TV: Survivors 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...) The Sontarans learnt of the imminent event, and the conflict it would provoke, from the assessment of Psychic Surveyor Kragar. (TV: The Halloween Apocalypse)

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 Inston-Vee Vinder's home planet, devastating it. (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. (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 event 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) Tecteun later revealed the Doctor's actions did stop the Flux from completely destroying the universe. (TV: Survivors of the Flux)

Aftermath of the Flux
The Flux had destroyed most of the universe, leaving Earth at the epicentre of what remained. (TV: Survivors of the Flux)

Whilst time ran wild, 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 where they rewrote history. 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, returning time to normal. However, the Great Disruption had lasted long enough to release particles of the Time Force which would erode whatever they touched. (TV: Once, Upon Time)

Some survivors of the Flux gathered on the planet Puzano, as it was the only planet even partly intact in its quadrant. They were visited by Azure who tricked them into believing she could take them to safety in another galaxy so they would willingly enter her Passenger. (TV: Village of the Angels) Swarm and Azure used these captives to strengthen the Time Force to create a psycho-temporal bridge to the Division ship outpost located outside the universe, where they killed Tecteun. (TV: Survivors of the Flux)

The Sontarans returned to occupy Earth weeks after their first attempt, now working with the Grand Serpent who had compromised Earth's defences from within. The Sontarans killed all of the Lupari in orbit, save Karvanista who escaped with the aid of the Doctor, and took their fleet for themselves. (TV: The Vanquishers)

Second Flux event
Swarm and Azure used the Division's technology to generate the planned second Flux event to wipe out the remaining universe, intending for the Temple of Atropos to be the last to fall, releasing Time. They planned to then rewind the destruction and play it out again repeatedly.

The Sontarans' psychic command, under the leadership of Kragar, used human psychics to predict where the Flux event would begin. Under the pretence of forming an alliance, they lured a Dalek and Cyberman fleet to the location, planning for their matter to resolve the Flux whilst their fleet sheltered behind the Lupari ships. The Doctor sabotaged their plan by helping Karvanista seize control of the Lupari fleet, whilst Vinder and Bel destroyed the command ship's communications to stop the Sontarans reacting in time. The Flux destroyed the gathered fleets of Daleks, Cybermen and Sontarans, however, this proved insufficient to stop it. Following a suggestion by Diane, the Doctor put Swarm and Azure's Passenger in its path and the matter within the endless prison proved enough to end the Flux. (TV: The Vanquishers)

Behind the scenes
The Flux acts similar to Davros' reality bomb, and is functionally similar to the Great Desolatrix, albeit on a much larger scale. 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.