I need help coming up with the motivations of some Dark Eldar antagonists. Aside from the standard knowledge, I don't have a all the ins and outs of this race figured out. That's where you all come in. First, I will lay out the details of the game I am running.

Game: It is based on the Rogue Trader rules/Tau PC Guide but heavily modified; it uses the DH2/OW/BC combat system as well as skill/talent descriptions. On top of this, we are also working with a nice list of house rules and homebrew equipment/gear/weapons etc.

Story: PCs are part of an elite Tau task force which currently consists of two fire warriors and one gue'vesa as well as a number of optional NPC companions to accompany them on missions; the latter of which helps fill out tactical roles the small group may be missing out on. They operate similar to how a Deathwatch kill team might - rolling around in a frigate under the command of a Shas'el taking missions as they come along.

Currently in the story, they have just started their first mission. A series of pirating raids in a system on the edge of the third sphere expansion have resulted in a number of lost merchant ships and a hefty amount of cargo - specifically aid shipments to colonies on the outer rim. A merchant vessel filled with Tau Marines and gun drones was set as bait on the edge of the system with its distress beacon activated to lure the pirates in in classic Kauyon style. PCs set up at the edge of the system in their frigate with stealth fields activated. Sure enough, raider vessel entered the system after a few short days of waiting and took the bait. They have a series of objectives to complete - primary, secondary, and tertiary - with plenty of mishaps in the works by yours truly.

The PCs infiltrated the open hangar via modified Orca gunship with stealth fields activated. They are currently in the middle of rolling initiative for their first fight after their not-so-sneaky Tarellian berserker and Demiurg operator were heard sneaking into the hangar control room.

Anyways, that is all background info. Onto the meat and potatoes:

A Dark Eldar ship is going to transit into the system as well. I have a vague story of how the chaos reavers and the dark eldar are in league with one another - a mutually beneficial raiding relationship - that each is expecting to crumble anytime. But I want to have the dark eldar involvement mean more to the larger, overarching storyline. Not just a one off. What would the DE be doing in the system? Obviously something sneaky and more nefarious than simple raids. Are they seeking out a necron tomb world? Are they looking for some lost relic that they heard was in the system?

I want to have the Dark Eldar ship get the hell out of dodge when/if they become aware of the Tau mothership. But why? I am having trouble coming up with some ulterior motivation for these baddies.

Secondly, what type of ship would you recommend the Dark Eldar be roaming around in? I would imagine it is nothing larger than a frigate - more likely something smaller.

Thanks in advance. If you have any other questions for me, don't hesitate to ask.

Canonically Dark Eldar and Chaos are very tenuous allies, so they are definitely not up to this. Tau are also aware of the difference between Dark Eldar and Craftworld Eldar, and absolutely despise the former (while being allies of convenience with the latter).

Perhaps the Dark Eldar approach the Tau masquerading as Craftworld Eldar to drive them against the Tau, then leave when they get a sense of the scale of the Tau presence, or else to report their deception so that they can drive it forward, use the Tau to crush the Chaos raiders, and then swoop in to kill whichever damaged side survives (ideally).

Dark Eldar are well thought of as unappolagetic evil. They know that they are not doing anything good, they know the culture they preserve isn't an ideal one (though it might be truer to the nature of the eldar than their self-denying craftworld-kin or the secluded and backwater exodites). But they won't hold back the feelings that grow strong in every eldar; pain, pleasure, dominance, superiority, excitement, battle. The craftworld kin supress their nature since it birthed Slaanesh, the Dark Eldar refuse to deny their nature and embrace the wickedness they have to blaze bright as they live.

That being said the Dark Eldar are not unreasonable, they often have hidden agendas when they co-operate with others but they do co-operate with others fairly often. They can be found as mercenaries, they can make alliances against a common foe, they can be negotiated with at times. They also still have respect for the other branches of the Eldar, they are the dark-kin for the craftworld eldar but still they are eldar and that makes them preferable as allies over most other species. (which is why they are battle-brother allies in the miniature game).

For their agenda here I would suggest them being allied with Craftworld Eldar to stop a prophesy (concerning Slaanesh) from coming true. Reveal the prophecy in parts to the players and have the dark eldar switch sides seemingly randomly (and eventually bringing in Craftworld eldar in the fray) as they work to stop different parts of it from coming true. This gives some mystery to the combat game and letting the players co-operate with extremely untrustworthy foes is sure to be an interesting scenario. This is of course only an idea and you can come up with another.