A gang war is the final fight for each London borough in Assassin's Creed Syndicate: once you have weakened the Blighters enough, their Templar leader accepts a street brawl between the two gangs, and winning it makes the borough Rook territory.
Finish enough bounty hunts, Templar hunts, stronghold raids and child liberations in a borough and a gang war becomes available. Meet the messenger to start it. The Rooks and Blighters clash in the street, and after the Blighters take enough losses their leader joins the fight in person. Beating them ends the war.
Whitechapel: Rexford Kaylock. City of London: Bloody Nora. Lambeth: Cletus Strain. Southwark: Octavia Plumb. The Thames: Edith Swinebourne. The Strand: Victor Lynch. Westminster: Lilla Graves, known as the Bludger.
Defeated Blighters defect, so the Rooks' numbers swell with each borough taken. The Gang Upgrades menu lets you spend money and materials to make them tougher in later fights.
Each leader brings something different. Cletus Strain uses smoke, Victor Lynch throws electrochemical bombs, Edith Swinebourne injects herself with a berserk dart, Lilla Graves is a former heavyweight boxing champion, and Octavia Plumb prefers to let her allies fight until she is forced to step in.
The first gang war, against Rexford Kaylock, is part of the main story and plays out differently: Kaylock puts his train on the line and you finish the fight on its roof while it moves.
Kaylock is knocked off the roof of his own train. The twins take the train as their mobile base and adapt his broken grappling hook into their gauntlets.
Gang Strongholds
Gang strongholds are fortified Blighter bases you raid in Assassin's Creed Syndicate. Clear one out and it turns into a Rook stronghold, chipping away at the Templar grip on that borough; each raid also carries optional constraints for full sync.
Somewhere That's Green
Somewhere That's Green is the first London memory: as Jacob you step off the train in Whitechapel, chase a pickpocket, race Evie up a factory roof and meet Henry Green, the Assassin who has been watching over the city. It ends with a carriage chase to shake off Blighters.
Sequence 3: A Modern Babylon
Sequence 3 brings the Frye twins to London and builds their footing in Whitechapel over five memories. You meet Henry Green, Frederick Abberline and Clara O'Dea, then take the borough from Rexford Kaylock's Blighters. It unlocks the A Modern Babylon achievement.