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I'm running a demo game at a local Con in February and would like to use a real fluff instance of when these 4 armies shared the same planet and fought against each other. I'd like to give the newbies a sense of background for the universe. Anyone have any fluff instances? or something close?

I don’t know of any canon where that has that happened, but it’s completely fluffy. Orks are having a good old Waaagh, and the marines try to intercept them at that planet, which happens to be a dormant tomb world. The Eldar step in to stop the Necrons from waking, but fail and it becomes a slugfest.

Edit: smart phone shenanigans

DoW3 if the expansion ever gets released.

You could use DoW Soulstorm as well. All 4 of them (and the other factions as well) fought over control of the Kaurava system.

The Kronus campaign from Dawn of War: Dark Crusade.

Or the latter part of the Lorn V campaign from Dawn of War: Winter Assault.

They were all on Cadia during the invasion, though the Space Marines and Eldar were mainly working together and the Necrons mainly fought against the Chaos incursion.

I'm sure you could make up some fluff of an Eldar craftworld that was disgusted of the Eldar working with the mon'kais leading to conflict on the battlefield. The Necrons didn't attack the Imperial and Eldar forces without being fired on first but I'm sure some jumpy Captain or Farseer might fire on space zombies on sight.

Several of the DOW games have this. DOW2 has some good campains also, but does not have all of them.

Weren't they all present during the Medusa V campaign?

In Overfiend, orks, marines, and eldar are all bouncing around the same planet. You could say some necrons woke up in the southern hemisphere off-camera if you'd like.

Or, for an off-the-top-of-my-head idea:
* Eldar foresee that a Waagh is going to mow through a cluster of maiden worlds and aim to redirect them towards some human-held systems. Marines move in to deal with it, forcing the eldar to intervene in order to keep the orks pointed in the right direction. Then the 'crons wake up as the fighting nears their tomb world and start trying to kick all the kids off of their lawn.

ALSO! I find it helps not to think of multi-faction campaigns/events as happening on one planet. That tends to get convoluted and busy really fast. Instead, think in terms of systems or sectors. The eldar might not care about a given planet that they're fighting on, but they do care about keeping the enemy from expanding towards the galactic north where their crippled craftworld is limping its way through the sector towards relative safety. The orks don't have to fight everyone on one planet, but they'll go out of their way to experience a sample platter of conflict across multiple systems. The marines might decide it's critical to save as many worlds as possible form the xenos threat and thus dispatch battle barges to every inhabited world in the sector. Etc.

All factions appeared on Medusa V with their own goals. Actually, it was quite similar to what they did in Dark crusade or Soulstorm.

In the 41st millennium there is only war. Deep in the Kaurava system the Orks have gathered their strength for a waaaagh. Aware of the upcoming invasion the Dark Angels have gathered several of their chapter on the planet Caspian III as a last line of defense against the brutish Orks. Unbeknownst to both forces Caspian III is an ancient Necron tomb world and their time of awakening is neigh. Knowing the threat awakening Necrons possess an Eldar fleet has arrived on Caspian under the stealth of night.

Which army will reign supreme and control the promethium refinery of Caspian III?

This is a demo game of 40k designed to help new players immerse themselves in the Warhammer 40,000 universe. 4 completely painted armies and a set of terrain will be supplied. Come fight it out over the fate of the Kaurava system!

Thanks for all your help guys!