Pala Olesis is the hidden homeworld of the Ronin. The planet's surface is 100% liquid (not including man-made settlements). All liquid on the planet is NH3 instead of H2O, thus presenting the questions of how and why Mankind settled the planet. It lies in the same system as the other planets of the Honorless.
Pala Olesis was settled sometime during the late Age of Progress by settlers sent out in later M3 in sleeper ships. The entire system was a checkpoint on the way to the Andromeda galaxy.
It was never rediscovered during the Great Crusade, and might not ever have been. It was by sheer accident that the Ronin found it in late M32 after the 3rd Black Crusade.
The planet consists of large sprawling metropolis cities. These are low rung and often partially underwater. As the planet was settled by people born in M3, everything was independently developed from mankind outside the system, with only sleeper and generation ark ships coming in every few millennium to bring more people and technology before the Age of Strife.
Pala Olesis is governed by an five member oligarchy. Each major quadrant of the planet elects a representative for life to lead. The fifth seat is held by the chapter master of the Ronin. This group must at least approve all the major decisions for the planet, but their main focus is on their interactions with other planets and groups. They are fine letting smaller governments run things on a day-to-day level.
Mankind was an ill-adapted species to live on a planet make of NH3 instead of H2O. But as some of the Imperial Guard forces show, humanity has a fairly flexible gene-code and can adapt. However, unlike most abhumans, you wouldn't know it unless you looked closely, where you would find iris-less eyes and skin with more yellow hues. And given that the only members that enter the Imperium proper are the Ronin, discovery of this is unlikely. These are the actual reasons for the acceptance problems in Ronin gene-seed.
The greatest defense of Pala Olesis is how obscure its location is, with only the members of The Honorless knowing its location. And even if they did, the system is well over ten thousand light years outside the Milky Way, so their travel times would be large. Aside from that, it is a fortress world, with large amounts of defenses developed independently. Massive pillars extending from each magnetic pole create, combined with the ones on the other planets in the system, a kind of Warp-hiding/killing field, preventing direct influence from Ruinous Powers.