Solifugid is a planet in Segmentum Obscuris that is extremely rich in adamantium, promethium and various other minerals. It is full of vibrant and exotic flora, many of which produce medicines and other useful commodities. The one problem with this planet is that it receives an inordinate amount of radiation from its sun, a neutron star. The plant life has adapted to this (which is why the products they produce are so useful) but during the day the surface is bombarded with many forms of radiation, excessive neutrinos, as well as dangerous heat. The Imperials have developed specialized bunkers in which to hide their workers during the day. Tunnels dug under the surface provide protection as well, though some of these are laced with promethium veins which is a danger in itself.
For unknown reasons, the Eldar have attacked this planet. In their wake came a small Tyranid hive fleet. The local PDF general could not determine why the two seemed to be acting in tandem. Later reports indicated that the Eldar were now avoiding the Tyranids. The general put out a call for help to the nearest Marine chapter, with the Emperor's Dragons responding.
Later, another chapter responded and the PDF commander's hopes soared. To his dismay, the green armored Marines were not the Dark Angels, but the Fallen themselves, led by Lord Cypher, chasing their own unknown goals.
Finally, the Imperial Guard arrived with a regiment of Cadians. A lengthy breakdown of communications saw the Imperial guard attack the Emperor's Dragons as though they were traitors. Stunned, the Marines were forced to fight for their survival, while trying to unravel the mystery of the communication failure.
Each faction vies for control of the planet's bunkers, for they hold the key to life on the planet. During the day, tanks and armor reign supreme while at night raids and ambushes keep the foes from sleeping. Underneath the planet's crust, forces clash in cramped, deadly firefights. During the confusing, the Tyranid Hive Fleet seems to have vanished, though auspex scans reveal unusually large fauna masses where none should be able to survive.
Ok, so the fluff isn't the best but this is the campaign my group is starting. I'll post rules on how we represent various things later, but this should serve as an introduction.