So I'm planning on running a game of BC for 4-5 players in a couple of months, and I started writing the story down a few days ago. Here's the basic premise:
The heretics start the campaign in a remote fortress city of a powerful chaos warlord. The city comes under attack by a rival warband of pirates, who manage to strike the city thanks to a man on the inside shutting down the fortress's defense batteries. The raiders are driven back, but manage to get away with both the traitor and a large stash of valuables. More importantly, the ruling warlord feels like he's been humiliated, so he hires the players to join his host in an effort to take revenge on the raiders.
The players will eventually take the fight to the raiders, slay their leader, capture or kill the traitor and recover the stolen property of the warlord. Among the stolen valuables is a piece of a broken, smoky black crystal-like object that seems to be calling for the players. Literally, with a demonic voice, or something like that.
It would begin to appear as though the raiders were after this object, which turns out to be a part of a much larger puzzle. The voice within the broken artefact promises that if the players bring together the scattered pieces of the artefact, it will grant them great power.
This is the problem: I can't come up with what the artefact would actually do once all its pieces are brought together. I figured there could be 5 or 6 of them in total, each located on a different planet or celestial object in the Vortex. It needs to be something relatively big, as many different factions would be after it - including possibly the remnants of the aforementioned raiders and the warlord (once s/he discovers the true nature of the artefact, and that the players have taken it for themselves)