The background for this event is directly linked to the BFG campaign I just finished up. The forces of Chaos have wrested control of the Denari system on the Eastern Fringe. Denari is a valuable Imperial resource planet in the sector and must be retaken at all costs. Control of the Promethium and Adamantium mining facilities are key goals to the campaign.
The lack of an Imperial Fleet presence has brought many forces to bear on the world in an attempt to capture its resources for their race. There have even been void skirmishes with scouting elements for a Tyranid hive fleet in the outer reaches of the sector.