ok i was wondering if the dark eldar would ever join the forces of chaos? Ive heared that eldar cant worship chaos or they die in an instant. could some one tell me if this is true or if its even possible for them to unite

none of the races would ever join forces in that sense, for any reason, apart from SM and IG (since they actually worship the same emperor).
However, they can probably team up "for the time being", in order to reach a common goal (i.e, get rid of a common enemy). But they'd still not have any love or understanding for each other. Thats how I understand it anyway..

Yes i know that much but has it ever happend

Can't answer that, sorry. I've never read about Chaos and Dark Eldar teaming up, but I suppose it's possible. Don't know too much of Chaos and DE lore, but followers of Slaneesh, I'd guess, aren't favorites of the DE. Then again, DEs are into the decadent way of life, hedonism etc, that gave birth to Slaanesh, so.. perhaps someone else can shed some light on this?

There was a battle report in white dwarf a while back with the dark eldar and chaos teaming up to attack an imperial settlement defended by the black templars. I think the base of it was chaos had their agenda and the dark eldar their slave fetish to satisfy, they united to further their own causes not for any lover for each other.

[Minor Spoiler]
In the Crimson Tears novel, a DE Warlord teams up with Slaaneshi cultists and sorcerers, so it has been done. DE worship Slaanesh anyway, in excange for him not eating their souls right away.

oh well thats new i thought if any eldar worshiped a dark god there soul would automatically be striped from the body

the DE don't worship slannesh per say, the reason they go to such length to live as long as possible at the expense of there slaves is to avoid being devoured b Slanneesh, unless something has changed in the new DE book I have yet to finish going thru all updated fluff.

As for DE and chaos getting together, yeah it been done, but they are both looking for a way to screw each other the whole time.

There have been temporary alliances between.

Eldar + Imperium (Various instances, sometimes space marine, However most of the time it has been either Rogue traders or Inquisition)

Harlinquins + Inquisiton (Guarding the black library from Ahriman)

Dark eldar + Chaos (A few times, Slaanesh chaos and Dark eldar sometimes enjoy each others presence)

Orks + Chaos (Khorne warriors mainly, there was an agreed to pact to fuck up an imperium base in an awesome battle)

Tau + Ultramarines (Tyranids)

Tau + Imperial Guard (can't fully remember this one, think it was against orcs)

Imperium + Kroot (Was in a Last chancers novel if I remember right)

Imperial guard + chaos (= Lost and the damned, Chaos does try and convert these people afterall)

..Sigh I really don't don't don't DON'T WANT to say this one..

Blood angels + Necrons (Against tyranids)

Ok this I have to read for myself..

Heretics!