So, I've been considering a campaign based around the premise that the Eldar try to deliberately give a Hive World over to Chaos so as to hamper the soon-to-awaken Necrons on said world. The Players get involved when they wipe out the cat's paw cultists and take the powerful, corrupting artifact.
Then the players have to work out why the Eldar are trying to hard to kill them, and the true nature of the artifact. Once that's done, they'll have to find a way to contain the Necrons, presumably by trading favours with the Navy, Space Marines, or maybe even bringing the Eldar around to help with the siege.
I'm excited about this campaign idea because I feel like it's something that could be executed in the background, without interfering with the freedom and self determination that Rogue Trader is designed around. The Eldar ambushes and the gradual revelation of the artifact can both be included in whatever the traders happen to do, and the investigating and favor-trading are both things that the players are pretty much free to take their own path in.
Although any comments and suggestions from more experienced RT GMs would be appreciated, I'm still particularly unsure about
1. What capabilities should the artifact have, in terms of things to make it A. appealing to the players, B. capable of spreading corruption on a massive scale, and C. really tricky to get rid of (Since the Eldar won't hunt the players if they just go "Oh no! It's cursed!" and throw it out the airlock)
2. How do I express to the players (who are largely new to 40k) the scale of the Necron threat, to encourage them to obtain as many resources as they can to combat it?
3. How precisely the Eldar interference should manifest- A Corsair raider? Warp Spider infiltrators? Ranger snipers whenever they go planetside?