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?
I am not an experience RT game master, but I am an experienced GM. So do with that what you will.
The Eldar hate Chaos as much as the Imperium does. The Eldar of the Expanse, according to the Warp Storm RT adventure series, have more reason to hate Chaos than most! How about a very large deposit (1000's ?) of soul stones on the Hive World, or a cluster of far seer soul stones, maybe in a (void) shipwreck in the planet's oceans? That might be something the Eldar would court the powers of Chaos over. You could just cut out the middle man, however, and have the Eldar awaken the Necron on the Hive World directly to distract the Imperium's attention away from a major Eldar excavation site. That defeats your wish of having a Chaos artifact and cult, however.
1) I recommend making the artifact unwieldy enough so that a single person can't carry it around inconspicuously. Maybe it is very large and heavy, maybe it makes constant loud wailing, weeping noises or exudes a strong stench of blood, smoke or sulfur? This way you have some control over its use by explorers. A.) Dedicate it to a particular chaos god and its powers follow: Tzeentch = a stable warp gate to somewhere important, Nurgle = Can cure / inflict any disease or perform resurrections, Slaanesh = Makes those nearby immune to fear or inflicts them horrible / pleasurable hallucinations (which are addictive?) and Khorne = makes nearby people belligerent driving them to berzerker rages. A good way to make ownership of the artifact enticing is by giving skill or combat bonuses directly to those who own it. For instance, an artifact to Tzeentch might give the navigator an +15% to Navigation: Warp tests. B. Corruption on a large scale is up to you. You can just say that everyone within a few hundred meters starts to change over a period of time. It could be hours, days, weeks or years. C. Making it large / cumbersome will keep it difficult to get rid of; if it's a 5m stone obelisk that weighs 3 tons . . .
2) This is a hard one. A new group of RT players might just abandon the planet and never look back (until it's too late, perhaps). I would recommend the hive world with more than one hive, each densely populated. Relate, while they are staying Hive B, about how Hive A was completely destroyed in a week (without the use of orbital weapons). That's 15 billion souls- dead! I'm not sure how you could appeal to flighty types, so that they stick around and fight. Maybe having the planet be the rogue trader's home world? That way a large percentage of the group's profit rating might be directly at stake.
3) I don't have much to suggest here. If keeping with my idea of an excavation site, I'd probably have a handful of warlocks and just plain old guardians and maybe a couple of wraithguard to protect the "diggers".
I'm not sure I can think of a plausible reason that the Eldar would try to attack the players (other than just being Eldar). It seems just as reasonable that the Eldar would contact the explorers directly and tell them about the artifact, so that the party would uncover it and bring about the Necron infestation themselves, which may also be a reason they would want / try to stop it.