Chaos and Dark Eldar Allies fluff

So, I've decided that I wish to ally my Dark Eldar with CSM, my only problem is finding an actual reason that they would form an alliance, both being incredibly arrogant and resource hungry, there would be no way that they would ally without a really good external reason

When reading the books it actually becomes clear that Chaos aligning with Kabals isn't toooo far off, however so far it never turned out any good for the DE being involved...

One could go for the "power hungry Archon" cliché who has a pact with a high demonic power in order to take over a hostile Kabal (perhaps even the Black Heart itself?).

Or an Archon who was slowly seduced by Chaos (especially Tzeentch if you think about it).

YoungArchon wrote:

So, I've decided that I wish to ally my Dark Eldar with CSM, my only problem is finding an actual reason that they would form an alliance, both being incredibly arrogant and resource hungry, there would be no way that they would ally without a really good external reason

Kabals striking pacts is not unheared of (just look at Vects battles against Kabals that allied with daemons).
It comes down to a pact with the devil where the Archon thinks (this is his very nature) he just outwits his bargain partner and can dispose of him/get rid of the pact after his "ally" did the dirty work for him.

You could always look at it the other way too- Maybe the CSM have sought the Kabal out.
In one of the Eisenhorn books, a human had struck a strong enough pact with the Dark Eldar that a Haemonculus was sent to assist the humans. I beleive in "Nightbringer" a human (formerly a prisoner of the Dark Kin) was released to further the Kabals aims as well.
Maybe the CSM need access or information that only the Dark Eldar can provide (Infiltrators, assassins, poison, psychic relics the Kabal can't use anyway) and is willing to trade slaves and/or souls for it, or maybe act as enforcers or ruin another Kabals Realspace raid. Dark Eldar are greedy.

I think I may go for the idea that they have a mutual agreement that they DE get the bulk of the resources, but the chaos get to convert what's left into a daemon world

It certainly isn't the most fluffy choice. Then again, this is 40k. Alliances are rare.

Best option could be slave trade, since that seems like something that could go both ways.
Another thing could be interrogation or other... stuff that they do to prisoners/slaves. I could see either a Kabal or a Coven (or a Cult) going for this. The Covens could be the ones most prepared for crazy scientist stuff. Fabius Bile was taught by a bunch of haemonculi, after all

Fluff wise I could see eldar of either stripe using Khorne as an ally simply because he is the most common combatant of their true enemy Slaneesh.

Beyond that logic however their is fluff in one of the books of Fabius Bile being trained by and having extensive dealings with the covens of the dark city.  It's where he gleans the technology required to deconstruct and tamper with the emporer's genetic "masterpieces".

So ya, the covens do eventually bring a gigantic reanimated corpse fortress through the webway to smash his army to bits and rearrange his brain.... but for a while they got along so it does happen.

You could always say that your CSM are captured space marines that have been twisted into abominations by Haemonculi. The reason they're not battle brothers is that the space marines are trying to resist their master's control...

Chaos and DE isn't totally unbelievable unless of course you have Slanneshi CSM then it's simply a big no no.

One idea that worked well for Dark Elves in fantasy was using the forces of chaos as sort of canon fodder, Malekith had Morathi strike a deal with the chaos warriors giving them riches, slaves, blah, blah and then used them as the bulk of his army so he didn't have to sacrifice any of his forces. Then once the invasion was done kill off any remaining chaos warriors.

This simple equation works well and is also highly translatable into Dark Eldar. Most believable would be Khorne because Khorne hates Slannesh. Of course I'd like to think Tzeentch managed to manoeuvre a few deals too.

The Cabal is a group of mixed, ancient xeno species who claim to have battled the forces of the Primordial Annihilator for a time longer than the existence of evolved humanity. They are chiefly known for involving themselves in the development of the Horus Heresy by using their foreknowledge of Horus' actions to seemingly suborn Alpharius Omegon and his Legion to their cause before the Great Betrayal took place.

I plan on running the hero's path Harlequin formation with the Fallen Champions from the Cypher Dataslate

Tzeentch would ally them then kill them "just as planned"
Khorne would do it to just make Slaanesh mad.
Slaanesh would do it for some - censors the rest -
and then kill them
Nurgle... Good ole Plague Daddy. I can't think of a reason he'd work work them. (super dirty and super Eldar clean?)

I do this, nurgle with DE, I fluff mine as a bunch of corsairs who have a pact - I suppose I'm more using the DE to represent a corsair fleet here though - just some thoughts.

There are a few instances of dark eldar allying with chaos. In one of the salamanders books, a chaos warlord has some sort of arrangement with an archon that gets the DE to attack and kidnap a world's population, eventually luring salamanders in to a trap. Another instance is in a soul drinkers book where an archon invites chaos to the party, using them to spread death, pain and despair to fuel his own agenda of a new commoragh...There are all sorts of reasons they could ally but generally they try to one up each other and eventually betray the other partner. I'm sure you can make exceptions to the rule if the partnerships proves profitable enough...say loyalist space marine captives for DE and chaos get the equipment (and the deaths of loyalists, they always like that).

I have been thinking about getting a horde of cheap cultist and using the renegades rules from Forgeworld as a human slave army.
I cant think of a reason to want to ally in CSM though. I have enough sub par units in my own codex thank you very much.

Always go with renegades over CSM cultists if your opponent is fine with FW - first off, they're cheaper after you consider the free wargear (assault grenades and autogun/shotgun upgrades).  They don't have to accept/make challenges, have a ton more options (like las/autocannons or melta guns/bombs), and can add on morale boosting options on top of a leadership that is already higher on average.  They can also field a wide array of supporting units from 20 point sentinels, to chaos spawn, to baneblades.

As for dark eldar using chaos cultists specifically - I think it's in the 3rd soul drinkers book - Crimson tears,

Don't click if you're about to read it and care about the "surprise" which isn't really surprising:

 These degraded humans pop in and out of the sewers with autoguns, or suicidally throw themselves out of web way portals on to unsuspecting enemies.

Mutant rabble could best emulate this as they have a 1 in 3 chance to gain acute senses and scout (outflank) before deployment, meaning a large squad of them could pop up in unexpected and annoying place in later turns.  You'd have to go in for multiple units though to guarantee that, and while they're cheap point wise (3 per with few upgrade options), You'd still have to buy a ton of models and paint them.

Fluff aside - there are a lot of things the dark eldar could benefit from in the renegades list.  Obviously a wall of AV 13 would be something novel for us, but what I usually find most useful (when using them as daemon allies anyway) are all the strength 6+ shooting attacks.  Even with lower ballistic skill (and HQ/elite units can be bs4), a dozen autocannons can force just about anything to try and stay out of sight.