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)

As the Flux progressed, it destabilised the Temple of Atropos on the planet Time, freeing the Time Force and releasing time particles, which caused near-instant decay in those it touched. (TV: Once, Upon Time) and Azure captured the survivors of the Flux by claiming to be their saviours, where they amassed on surviving planets, such as Puzano. (TV: Village of the Angels) The final Flux event would mean the end of everything, designed as an opportunity for the Division to pass over into Universe Two, leaving nothing behind. Swarm and Azure took power from Division Control. (TV: Survivors of the Flux) However, the Thirteenth Doctor was able to avert the end, and Swarm and Azure were taken by Time when they attempted to sacrifice the Thirteenth Doctor, having failed at their task. (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". (TV: The Flux is Coming...) The Sontarans learnt of the imminent event, and the conflict it would provoke, from the assessment of Psychic Surveyor Kragar. They believed it would dwarf every previous conflict in history. (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 control console. (TV: The Halloween Apocalypse) While the interactions of energies stopped the first Flux event from completely destroying the universe, (TV: Survivors of the Flux) it caused several system malfunctions in the TARDIS and caused the ship and its occupants to fall through time. (TV: War of the Sontarans)

Immediate aftermath
The Flux had destroyed "many galaxies", leaving Earth at the epicentre of what remained. (TV: Survivors of the Flux) Tecteun would however note that the Doctor disrupted the Flux event "just as it came into existence." (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 erased the Sontaran invasion 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 that she could take them to safety in another galaxy so that they would willingly enter her Passenger form. (TV: Village of the Angels) Swarm and Azure used these captives to strengthen the Time Force to create a psycho-temporal bridge to Division Control 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)

Final 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. At the same time, at the request of the Doctor, the Division's Ood did his best to minimize the impact of the Flux so as to make it easier to stop. The Flux destroyed the gathered fleets of Daleks, Cybermen and Sontarans, however, this proved insufficient to stop it and some of the Flux was able to make it past the Lupari shield. 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 with Time describing the Flux as being "extinguished". (TV: The Vanquishers)

One week later, Dan stated the Doctor had "saved" the universe through her actions. (TV: Eve of the Daleks)

Behind the scenes
In terms of comparisons to real-world scientific theories, the Flux appears to have many similarities to the theory of, which describes an event which would erase the laws of nature completely. In the DWU, the Flux acts similar to Davros' reality bomb, and is functionally similar to the Great Desolatrix, albeit on a much larger scale. (As for comparisons to wider pop-culture, the cataclysmic event it causes is similar to DC Comics' Crisis on Infinite Earths and the Nothing from The Neverending Story.)

Following the release of The Vanquishers, there has been some degree of confusion as to what the exact resolution to the final Flux event is within the Doctor Who universe. A Dalek Executioner in Eve of the Daleks does refer to the fact that the Dalek War Fleet was destroyed in the Flux, with their mission being to hold the Thirteenth Doctor accountable for "the destruction of millions of Daleks". Thus, clearly indicating that the destruction of the Dalek, Sontaran and Cybermen fleets that occurred before the deployment of Passenger was not undone.

The less speedy and less destructive nature of the final Flux event may also be attributed to the Doctor's interference alongside the Ood stationed at Division HQ as in The Vanquishers, the Doctor asks the Ood about their ability to "minimise the final Flux event". (To which said Ood vows to try their best and later says that they "have done all [they] can".) With regards to both Flux events, Dan Lewis notes in Eve of the Daleks that the Doctor had "saved the entire universe last week".

This is supported by the Doctor's reference in Survivors of the Flux that the damage from the first Flux event "can be decompressed, uncompressed and I'm thinking, with those transport pods, this power source, reversing the polarity of the conversion plates, I can stop this."

What is less clear is exactly how large the impact of the first Flux event had actually been in the first place. In Survivors of the Flux, the Ood at Division HQ notes that the event had "destroyed many galaxies." By contrast, Tecteun remarks that the Doctor "[disrupted] the Flux, just as it came into existence."