Great Disruption

The Great Disruption was an event in which the Ravagers, Swarm and Azure, successfully disrupted the flow of time by taking command of the Temple of Atropos, on the planet Time. Since all time ran through the Temple, this impacted the whole of the Doctor's universe, creating incoherence, and unleashing the Time Force, which had been constrained.

This force saw its way into the universe through particles which caused erosion and decay, turning everything they touched to nothing. Many people throughout the universe fell victim to these particles, which behaved like active predators, approaching moving targets in proximity. Meanwhile, time ran wild, with no apparent sense to the passage of time: day turned to night in an instant, and then back again.

From within the flow of time, anchored to Atropos, the Thirteenth Doctor was able to restore Time with the help of the Mouri, convincing them to send two of their number to replace those that Swarm had destroyed. Despite this, the damage had been done. Swarm declared that they had succeeded in their mission. The Time Force particles remained, even after the Temple had been restored. (TV: Once, Upon Time)

Origins
Like the Flux, which destroyed the very structure of space as it advanced, the Ravagers' disruption to the very flow of time was by design, and would bring an end to this universe, as planned, if left unchallenged.

The Thirteenth Doctor discovered that these plans had something to do with her long-forgotten past, specifically relating to the period in which the Fugitive Doctor was still in service to the Division. (TV: Once, Upon Time)

Consequences
Emerging from quantum disruption, the Time Force brought particles into the universe which could reduce matter to nothing, instantaneously eroding whatever it touched. According to Swarm, this was a process called temporal destruction. These particles attacked many people throughout the universe, destroying them on contact, though some, notably Bel and Joseph Williamson, managed to evade them. (TV: Once, Upon Time)

Even after the disruption event had been resolved, with the Mouri re-instated, the Time Force particles remained. (TV: Once, Upon Time)