Ynnead

Ynnead looking like he's about to wreck Slaanesh's shit. Also that mono-boob screams irony.

OOOOOooooooOOOOoooooooOOOOoooo.....2Spoopy4Me.... - Ynnead pondering on his existence

“To the last, I grapple with thee; from hell's heart, I stab at thee; for hate's sake, I spit my last breath at thee”.
— Captain Ahab, Moby-Dick

Ynnead is the Eldar God of the Dead Voltron ... or at least he will be, as he is currently being made. He is currently in godly limbo at the moment as the Eldar try to get enough pure souls or at least desireless souls together to give him enough mojo to form together into a proper god.

The whole process of god creation is a bit of a tricky one to explain but we'll try to keep it simple.

Basically, Ynnead is the Eldar's last hope against Slaanesh. When the bitch-goddess was born she devoured a lot of Eldar and their souls and most of their gods and basically caused a lot of things to get badly screwed up, making a terrible mess in the toilet afterwards that the Eldar where forced to mop up. To add further salt into the wound, the surviving Eldar found Slaanesh was constantly hungering for their souls, laying in wait for the moment an Eldar would die and then he would completely suck them off dry. In the past the Eldar believed when they died, their soul would be reincarnated in the form of a Jokaero. With Slaanesh's coming though, the Eldar's ability to reincarnate disappeared.

To save them from this fate, the Craftworld Eldar use a spiritstone to trap their soul inside upon death. The spiritstone is then placed within the infinity circuit next to a carefully placed egg timer of the chosen craftworld, where the soul is free to roam around and commingle with the other Eldar souls of the craftworld.

As more souls over time have been added to the infinity circuits, the Eldar Farseers have perceived something stirring from this collection of souls; this is Ynnead, a god formed from untainted Eldar souls and the power of Asuryan, the king of the Eldar gods, who passed on his power to the Farseers of the Eldar before he was devoured by Slaanesh. The Farseers believe that when the last Craftworld Eldar dies, Ynnead will be fully born and will rise up to cast down Slaanesh, destroying him forever.

Whether this could be true or not, considering Slaanesh's immense power, is unknown, but Ynnead is the best hope of the Eldar for a better future for them and so all their efforts go towards making him as strong as possible. Plus, Khorne would probably jump at the chance to finally get rid of Slaanesh and its merry band of chickenshits once and for all, consequences be damned.

Assuming Ynnead gives a shit about doing that, that is. The Eldar's past track record for creating gods makes it highly dubious that this one will turn out any better, and as far as anyone knows it could just make like Nagash and try to make itself a Chaos God instead.

The Star Child / Ynnead theory

The theory stating that when the Emperor screwed Horus's soul to the wall, part of the Emperor's soul was also cast into the Warp. This Emprasoul fragment is the Star Child, another god waiting to be reborn - or perhaps be reincarnated into human flesh. If the remains of the Emperor were ever to die, the tiny spark of soul left in his body would re-unite with the greater whole, and according to prophecy, stalemate the four Chaos Gods while the races of the galaxy were left to battle it out in one last great Ragnarok scenario.

However, more recent (2006 onwards) fluff from Xenology puts a glimmer of hope on this theory by suggesting an image of an "unborn child surrounded in starlight" as one of the three future survivors of the galaxy's conflicts (the others being Khaine and the Laughing God). While it can be equally suggested that this is far more likely to refer to Ynnead, who is the Eldar god that shall be born from the webway when the last Eldar has died, it is now becoming possible that Ynnead and the Emperor's star child could be the same thing, particularly with the revelations in Horus Heresy that the Golden Throne is a doorway into the webway itself.

This is made more relevant when you consider the fact that the Eldar Gods (the Old Ones) do not belong exclusively to the Eldar alone, and were active amongst several races all at the same time. So Ynnead does not have to be an "Eldar" god, just a powerful enough being to actually count as a god. The Emperor ticks that box, finally unifying the two species in the galaxy with the most in common, and, truth be told, the least reason to fight (except for the Dark Eldar).

On the other hand, the Star Child theory's biggest proponents proved to be cultists of Tzeentch, which makes it very likely that all of the above is completely irrelevant. So believe what you will, and always act on the assumption that most of it is wrong somehow.

The Gathering Storm

It appears Ynnead is up and active during the 13th Black Crusade, seeing as how it now has an Avatar!!! (Oddly, or maybe logically in retrospect, considering Ynnead was created after the Eldar became 'connected' to She Who Thirsts, the Avatar appears very Slaanesh-like, with a large horn, as well as mono boob syndrome similar to some daemonettes and depictions of Slaanesh). Accompanying it are Lady Maly- I mean Yvraine, an emissary (who is pretty "thick" for an Eldar), as well as Visarch (a former Incubi brought back to life by Ynnead).

The Gods of the Eldar
Asuryan - Cegorach - Isha - Kurnous - Khaine - Lileath - Morai-Heg - Vaul - Ynnead