This encounter has clearly devolved beyond my expectations.

Those who claim that the Eldar cannot win bear the same arrogance they attribute to the elder race. It is true that the Eldar do not possess the power or capability to wage war against the Ruinous Powers alone, however, believing they are alone in this fight is simply delusion.

Those who claim the Eldar cannot lose are merely adding fuels to the fire. Was it not the words of your greatest Farseer that implied an uncertain future? Belief in certainty grants hope, will and determination, in this I cannot judge the attitude as a flaw, but it would take one truly consumed by hubris to suggest it does not bear inherent weaknesses.

Humans, brothers and sisters of Chaos, Eldar, Orkz. The End Times are soon to be upon us, prepare yourselves for this moment or be consumed by the fires that render even the greatest of mortal and Daemon alike insignificant.

what story

my interest is peaked, do tell

I stand by my belief that Ynnead cannot win.

That doesn't mean Slaanesh can't lose thou.

I guess that all depends on what you consider as winning? Win a direct fight against She Who Thirsts? No. Defeat She Who Thirsts using Eldar guile? Yes. Don't forget Cegorach will play his part in this...

Also depends on what "stratagem," Ynnead decides to take in sustaining itself. Sure, to begin with it will be made of all currently preserved Craftworld Eldar souls, as well as most Corsairs (which wouldn't come close to a Ruinous Power in raw energy, but would still be phenomenally powerful), but there's nothing to say it couldn't gain more from somewhere, it would just have to do it quickly, otherwise Slaanesh will eat it.

Gotta remember that the Emperor of Mankind is basically a "Ruinous Power," at this point too, weaker than the other four, potentially, but still incredibly strong. When his material anchor is severed he'll also give the Chaos Gods a hard time.

If Ynnead came about by itself? It would stand no chance and have a lifespan equating to a few minutes for a God. The thing is, Necrontyr are reactivating, generating null-fields and generally weakening the Ruinous Powers a little bit. The Tyranids are consuming or otherwise blocking off many Human systems, relieving Chaos of some of it's food. The God Emperor of Mankind represents a serious threat to Chaos, not enough to destroy them but still threaten their stability of control.

With all of these combined, adding Ynnead on top? Yeah, the Chaos Gods should definitely feel threatened right now. They have the capability to stop these threats, but if they don't they're gon' lose.

tl;dr, basically everyone is knowingly or unknowingly ganging up on Chaos right now, with the exception of non-Imperial Humans, Tau and Dark Eldar.

I think you may all be forgetting that Ynnead is not simply a god accidentally born as was Slaanesh - but also bears some unclear relationship to Kaela Mensha Khaine. Currently Khaine is shattered across the stars, but given any Eldar may don the role of the Young King, and become an Avatar of Khaine - it stands to reason Khaine now lies within the living Eldar: this is how he survives Slaanesh.

"Once an Exarch is chosen, he or she becomes the Young King. The gender matters not, and a female will still be referred to as the "Young King", for she embodies Eldanesh, and Eldanesh was male. When the Ritual of the Young King reaches the moment for awakening the Avatar, the other Exarchs will gather in the antechamber of Khaine's shrine and form a circle around the Young King, softly chanting. They will solemnly assist the Young King in removing his or her Exarch Armour, which will reverently be returned to its Aspect Shrine. Once this is done, the Young King will stand tall, completely naked, arms spread wide and head bent backwards in adoration while the most senior Exarch of each Aspect carves the rune of its Aspect of War in the Young King's back. On some Craftworlds, a golden cup called the Chalice of Criel is held to the small of the Young King's back to collect his blood, while on others, it is allowed to flow freely, anointing the ground of Khaine's shrine. Once all the runes have been carved in the Young King's back, the Exarchs will bring forth the Suin Daellae, the Howling Death, the gigantic spear or sword that serves as the weapon of the Avatar of Khaine. The Young King will take the Suin Daellae while the remaining ornaments are brought and placed upon him; most often these are a cloak and a simple garland of flowers. If the Chalice of Criel is used, it will be presented to the Young King who will take a sip of his own blood before accepting the cup. Once the preparations are completed, the doors to the inner sanctum of the shrine will be opened, and the Young King will step forth carrying the Avatar's weapon. The doors will close behind him and the Exarchs will offer a hymn of praise to Khaine and thanks to the Young King for his or her sacrifice. What exactly happens in the inner sanctum, none knows, for no Young King has ever lived to speak of it. But inevitably, after a few moments, the doors will open again, and the Avatar, fully empowered, will march to war. Of the Young King, no trace, save perhaps the dripping blood on the hands of the Avatar, will remain..."

Therefore Ynnead is not only culminated from the careful craft (via Paths) of psychic will being fed into the infinity circuits of all the living Eldar for the last 10,000 years - but also by combining all our souls to birth Ynnead, we in the same act are reconstituting Kaela Mensha Khaine: in some way or other (their relationship is yet unclear).

We are not simply making an equal to Slaanesh, for our god-craft has improved significantly since we accidentally made Her - but we are also arguably feeding a God of War to Ynnead along with ourselves. A god without equal in combat amongst the Eldar pantheon - who has killed C'tan gods in the past, something even the Old Ones could not do - and who (depending on canon) may at this point be in some way be psychically connected to Khorne himself.

So let us be clear - we Eldar are not fucking around. At the very least we are improving upon the process that made Slaanesh - a ruinous power - in the first place. Very likely we are also feeding a god of war (Khaine) to this new god at the same time. Potentially we are also sacrificing Khorne to Ynnead too. In short, if we succeed in virtually any of this - Slaanesh is fucked.

It also explains why the Mon'keigh are right not to trust us - because while we seem like a relatively aligned ally (in that we both dislike Chaos and Necrons and Tyranids and shit) - we make the recklessness of unsanctioned psykers look completely trivial - because we a) already made a ruinous power before, and b) are potentially planning to sacrifice two gods of war to make some sort of super-god-eating-god: because that couldn't possibly backfire on the universe

It's the sort of thing where even the Eldar have to look at our grand plan and wonder if this isn't the worst idea ever - but it's totally cool because our Trickster God of plots and deceit told us it was a great idea

That's the fun of being Eldar though, we get to be arrogant enough to feel confident in what is almost certainly the Worst Idea Ever

Where is that write-up of the Young King from? That's quite poetically written

If memory serves it's from Rogue Trader. But I may be wrong. Forgive me if I am, my copy is in storage so I can't check.

And that's why it will go to plan. Now, is Ol' Ceggy's plan is actually the plan that he's shared with the Eldar? That's another question all together...