While background is not my expertise, I still enjoy reading through it as the Warhammer 40k universe is extremely detailed. The history and background of Mars is one of those interests that a lot of people have and are looking forward to reading when this new codex comes out.
If you do not like spoilers.... dont read further
Please remember that these are still rumors, as the book has not been released. Just the hint of Dark Eldar in the book, means I'm hooked, and need to know more.
via anonymous sources on Faeit 212
Not much im afraid, the background -timeline- starts at the end of the horus heresy saying that the Mechanicum was reforged as the Adeptus Mechanicus in M31. The codex has a very brief timeline in it that includes a shadow war with the Inquisition, and tech-priests finding failures in the emperor's throne resorting to a bargain being had beneath the city of Commorragh.