I guess I'll do it, but first I'd like to know what you guys think of this story for it:
The planet of Proxima II, once a small, largely barren world, whose only redeeming factor was its system's proximity to Alpha Centauri, had begun gaining considerable renown among the Imperium as a terraformation attempt by a relatively successful business on Alpha Centauri IV reached its final stages of completion. This business, realizing the relative lack of safe interstellar tourism venues in the Imperium, began terraforming Proxima II as a high-class vacation planet situated safely in the heart of the Imperium. However, after many years of very successful business, strange occurrences began to erupt throughout the world. Tales of sudden bursts of murderous rage among seemingly perfectly stable individuals, of strange mutations among local flora and, most troubling of all, the loss of communication with a small, remote luxury resort on the far side of Proxima II's moon and the disappearance of all expeditions sent to investigate this. All of these occurrences were, of course, covered up by the company which owned the planet, being bad for business.
It was not until communications with the planet began to be mysteriously jammed or garbled, along with a sighting of peculiar, leaping aliens leaking out that the Alpha Centauri militia commander, Commander John Taurus, decided to investigate. His arrival was heralded with hordes of executives bearing bribes worth billions urging him to leave with shining reports of the system, all of which he rejected. It took only a single pass of his ship's scanners to reveal a massive, tendril-like cloud of debris stretching millions of miles from the planet's surface falling down upon it. While attempting to investigate further, however, the commander's fleet sent out only a single distress signal before all communications were suddenly silenced.
It was then that the Imperium knew something was definitely amiss on Proximia II, and its closeness to the very heart of the Imperium, the Sol system, ensured an entire fleet of Strike-Cruisers, carrying a full legion of Space Marines, were quickly dispatched to investigate.
Allowed factions:
Imperial Guard
Space Marines
Chaos Space Marines (What could these guys be doing here, I wonder?)
Factions I could possibly add with enough demand:
Eldar
Dark Eldar