Fight clubs are underground boxing matches around London in Assassin's Creed Syndicate, run with bookie Robert Topping taking the bets. You fight several opponents at once with your fists, and winning pays out money.
Matches look like boxing bouts, but you face more than one opponent at the same time, so crowd control and counters matter more than in a one-on-one.
Twenty years on, in the Jack the Ripper expansion, the Rooks under Jack run fight clubs in Whitechapel and the City of London where fighters die in the ring. Chief Inspector Abberline asks Evie to shut them down; the Mitre Square club is one of them.
Robert Topping was the fifth of eight children and spent a stretch of his childhood travelling with a circus after falling asleep in a crate that ended up in its stores. Back in London he worked for an uncle who ran shady businesses. Evie first finds him tied up by Blighters at David Brewster's laboratory in Croydon, and he slips his bonds with tricks learned from the circus. When the twins meet him again in London, he explains his place in the underworld and invites them to enter his events.
In the expansion, Mitre Square's club in Whitechapel had been closed for years before someone reopened it, and it draws a big crowd thanks to a busy district and a nearby train station. Cock Lane's club in the City of London is hidden in a pub basement and caters to wealthy gamblers. Both memories share the full-sync goal of frightening 15 enemies, and each ends with Abberline thanking Evie for taking another criminal off the street.