Null Sector was an omnic extremist group formed in London.
A large amount of England was built by omnics working in hard labor conditions. In spite of how much they contributed to society, omnics were treated harshly in the United Kingdom, being denied basic rights and citizenship. Omnics were also treated as though they were criminals or under military occupation, such as being forced through special checkpoints to be scanned, or being marched through streets with armed soldiers close by. Null Sector was formed as a response to this harsh treatment, their goal presumably being to achieve equality between omnics and humans - by force.
Null Sector was supported by local omnic population unhappy with their treatment, and would not hesitate to attack fellow omnics if they were defending the humans. They had great power at their disposal - B73-NS Bastion units, upgraded OR14 units, and more. As time progressed, their control over London increased.
Seven years before the present day, Null Sector launched a surprise attack on King's Row in London, taking the city and killing hundreds in the process. On the 28th day of their uprising, the Overwatch agents Mercy, Torbjörn, Reinhardt and Tracer were deployed to King's Row (without the explicit concent of the british prime minister) in order to prevent an attack on the power plant, and put an end to the organization. They fought through many troops, and were ultimately successful in their mission.