In M41.999, Messiac was a planet in Segmentum Tempestus. It was a Proto-Hive, with a decently sized population.
However, on the day of the Fall of Cadia, an Adeptus Mechanicus base on the planet failed with their highly heretical experiment with xenotech, after the cohort of Adepts was infiltrated by an unknown entity.
After the ensuing chaos of the facility being overrun by Chaos Daemons, a handful of Adepts survived both the Chaos incursion and a Scion organization dedicated to quelling said incursions...and those who opened the portals.
Then, the Great Rift opened, and the world was slammed by waves of Chaos. With the enclave of Humanity on the planet barely surviving the plentiful invasions of Chaos, it was much less prepared to deal with the Culture Complex, a new xenos faction transported from their location in the Fringes by the chaos of the Rift.
The CC quickly overtook Messiac, and using their strange xenotech they ruled the planet. With the last loyalist forces now a Resistance, they fight to reclaim their world from the vile Culture Complex. But not all humans remained loyal to the God-Emperor, many were indoctrinated and forced to become the CC's soldiers. Traitors, perhaps, but the only truth the Resistance has is that they fight for the CC, and must be slain all the same.
The Messiac Resistance is only ten thousand strong, if that, but their salvaged equipment, along with Mechanicus-sanctified CC weapons, is enough to kill CC soldiers and even the xenos themselves. With a single Space Marine and a hero from the Mechanicus base rallying Humanity against the CC and successfully destroying the xenotech portal tower, for once, the future looks bright for Messiac. But the fight is not over yet, for the CC retains its grip over Messiac. Only once they are forever pushed off the planet will the Resistance stop their heroic crusade.