https://www.pcgamer.com/overwatchs-uprising-event-returns-in-april/

The Overwatch Uprising brawl will return on April 10, Blizzard announced today, taking players back to "a pivotal moment in Overwatch history" when the team fought against rampaging Omnics in the city of London. 

The King's Row Uprising took place seven years before present-day Overwatch, when an extremist group of Omnics calling itself Null Sector launched an insurgency in an upscale neighborhood of London. Casualties and destruction ran high before the Overwatch team—prior to its dissolution—got involved. When it was finally over the whole thing was buried deep in Overwatch archives.