We have a rebellious commander possibly possessed by a chaos artefact sitting in heavy fortifications just outside our space
Actually, we have no idea what exactly the Dawn Blade is, other than its a non-Tau artifact. It could be a Chaos weapon just as easily as it could be a weapon of the Necrons, the C'tan, the Eldar, the Old Ones, or a forgotten, lost race, just to list a few possibilities. In fact, in the old GW website, there was an article where they two Farsight conversions, one where Farsight was ultimately still loyal to his people and the Greater Good, merely carving his own path without the Ethereals, while the other had the Dawn Blade as a C'tan/Necron artifact (I can't remember which, but you get the point) and he and his warriors becoming unwitting warriors of the C'tan.
Nor do we know what exactly has befallen Farsight and his Enclaves.
Personally, I'm a Tau fan, and tend to be more on the optimistic side. I hold out hope for Farsight and his warriors, that they may be rogue, but ultimately, they're still loyal to their people. And I tend to favor the hopeful notions of 40k and its potential future. But that's just me. I want the heroes for fought and died, to have fought and died for a reason, for something better than what currently is.