The galactic government that runs the crawl and now faces open, universe-wide insurrection from its former contestants.
The Syndicate presides over the crawl and everything around it, insisting that even the exit negotiations after Faction Wars be conducted by the books - which is why Quasar is a volunteered lawyer sitting in a Zero Zone cubicle offering Carl deals that are, by his own account, all shit. Roughly twenty thousand crawlers survived to the tenth floor, and the sheer supply of survivors is what makes the offers worthless. Syndicate courts, the Syndicate Intergalactic Bar Association and the The AI still hold the paperwork together.
It is also the enemy that killed York's hive, and by the epilogue it is under attack from several directions at once. Porthus broadcasts a declaration that a new militant group will move against all Syndicate governmental properties and systems across the known universe unless the crawl is stopped. Syndicate security corvettes in Earth orbit are outgunned by a single behemoth carrying weapons they never bothered to mount. A new Prime Minister, a crashed The Dream cruiser, a mass suicide in the Nebular colonies and the destruction of a mantis system fill the news feeds simultaneously, and an embargo has strangled traffic through the Earth system's tunnel gate.
The ruling power of the crawl, now the declared target of a galaxy-wide resistance campaign.
Also appears in books you've already read — spoiler-safe to their endings: