Since there are already former Imperials who defected to the Tau empire, would it be plausible that a remote Space Marine chapter could rennounce the Emperor in favour of the Greater Good?
The way I imagine it:
A chapter is founded on a remote world just on the edge of the Tau empire as a defensive line if the Tau should start a military initiative.
Communications are cut off by a warpstorm for a few years. After contact is restored to the rest of the Imperium, the inquisition gets suspicious of mutations and send Grey Knights in to purge the chapter. After some hard-fought battles that they barely survive, the chaptermaster decides that the Emperor is no longer worthy of their services, since his hounds betrayed and attacked them. But they also don't want to turn to Chaos.
So after soem existential crisis in which they basically stay neutral, some scouts on their home planet come across a crashed messenger probe from the Tau. They are intrigued by a dogma that doesn't involve any deities and decide to join the fight for the Greater Good.
Their actual strength would of course be very limited, since they can only recruit from Gue'la, and Tau command would distrust them because of their barbaric methods and overzeal, keep their alliance discreet, only using them for the nasty missions and so on.
I'm still not clear on the details of the chapter. I'm probably gonna have their founding chapter be the White Scars, because I wanna go with a kind some similar theme, but using Ancient China instead of Mongols (saffron and light orange gear, demon mask helmets, dragon motifs and ornaments on armor) and basically the same army list-up, concentrating on fast vehicles but also lots of sniper scouts thrown in to make them feel a bit more Tau-y.
The way I imagine it:
A chapter is founded on a remote world just on the edge of the Tau empire as a defensive line if the Tau should start a military initiative.
Communications are cut off by a warpstorm for a few years. After contact is restored to the rest of the Imperium, the inquisition gets suspicious of mutations and send Grey Knights in to purge the chapter. After some hard-fought battles that they barely survive, the chaptermaster decides that the Emperor is no longer worthy of their services, since his hounds betrayed and attacked them. But they also don't want to turn to Chaos.
So after soem existential crisis in which they basically stay neutral, some scouts on their home planet come across a crashed messenger probe from the Tau. They are intrigued by a dogma that doesn't involve any deities and decide to join the fight for the Greater Good.
Their actual strength would of course be very limited, since they can only recruit from Gue'la, and Tau command would distrust them because of their barbaric methods and overzeal, keep their alliance discreet, only using them for the nasty missions and so on.
I'm still not clear on the details of the chapter. I'm probably gonna have their founding chapter be the White Scars, because I wanna go with a kind some similar theme, but using Ancient China instead of Mongols (saffron and light orange gear, demon mask helmets, dragon motifs and ornaments on armor) and basically the same army list-up, concentrating on fast vehicles but also lots of sniper scouts thrown in to make them feel a bit more Tau-y.