So, not an original theory, but one I'm hopefully not just repeating blindly.
The current known Croneswords are as follows:
Kha-vir, the Sword of Sorrows - The sword of Yvraine, turns its victims into ash[2]
Asu-var, the Sword of Silent Screams - Resided on Biel-Tan. Absorbs its foes life essence. Now wielded by the Visarch.[2]
Vilith-zhar, the Sword of Souls - The largest and most powerful blade. A shapeshifting blade forged in burning souls. Recovered on Belial IV and now wielded by Yncarne.[2]
The Spear of Twilight[2]
I want to draw particular attention to the English names of the weapons, styled in the pattern "The XXXX of YYYY."
Of all these, the Spear of Twilight is the unique one, as it has no Aeldari name counterpart now, but also being in an alternate form.
Lexicanum wrote:
Yvraine reverted the Spear to its true bladed form
I fine it very interesting due to the use of the word "bladed" instead of "sword." Based on the model, and the fact it does have a blade as a spear, why mention this word? If it already has a blade, why not simply say "true form?" Again, if it were transformed into a sword, why not simply say "true form of a sword" or similar?
My theory is that this word is draw specific attention to the English name of the weapon. Rather than calling its new form "The Sword of Twilight" but the "Blade of Twilight" or "Twilight Blade" This would make it a theoretical and powered opposite to Farsight's Dawn Blade, or more connectedly, the Blade of Dawn. The Spear of Twilight is known that it will eventually burn out the life of the user. In other words, it shortens the lifespan of the wielder relative to use. By contrast, the Dawn Blade lengthens the life of its wielder relative to use. This parallel makes it almost certainly the polar opposite of Yriel's weapon. It also leads to a parallel between their wielders. Yriel struck out from his world and became a raider and outcast, because he did not agree with the status quo but ultimately return to save his people. On the reverse, Farsight was cast out because he knew too much and challenged the status quo. The Tau and Eldar are opposite in many ways, psychically, age wise, lifespan, the status of their Empires in the Twilight and in the Dawn respectively.
However, there is more to this theory.
Deep within the Black Library, exists several tomes. One of these was intended to be the Stern Codex: a book written by a sister of battle that supposedly contains all the info on Slaanesh, strength and weaknesses. The book and sister were last seen travelling with an Eldar into the webway, heading for the Library. Another is the book of Rhana Dandra, the Eldar Extinction. According to newest lore, it finally opened after the Great Rift, completely changing the story of the Fall of the Eldar.
Instead of the Rhana Dandra, the final act tells of Cegorach's ultimate jest that would trick Slaanesh into expending all of her energies to save the Eldar instead of destroying them. How such an event could come to pass remains unclear.
What has this to do with the Croneswords? Everything.
The final act of the book of Rhana Dandra does not explain how this final act, where Slaanesh destroys herself to save the Eldar. This implies a missing page, so to speak.
The Croneswords are said to grant power over life and death, but also resurrect Ynnead. The important thing, however is that their powers and names provide this missing page to this tome. They provide a step by step instruction book as well as a story, on how to defeat Slaanesh.
Kha-vir, the Sword of Sorrows - The sword of Yvraine, turns its victims into ash - this blade tells of how Slaanesh was formed from the ashes of the old Eldar pantheon and empire, and the sorrow of the Eldar as a result.
Asu-var, the Sword of Silent Screams - Resided on Biel-Tan. Absorbs its foes life essence. Now wielded by the Visarch.[2] - this sword tells of Slaanesh devouring the souls of the Eldar, silently screaming in the warp and absorbed by she who thirsts.
Vilith-zhar, the Sword of Souls - The largest and most powerful blade. A shapeshifting blade forged in burning souls. Recovered on Belial IV and now wielded by Yncarne - this sword tells of Slaanesh at the height of her power, made from the burning souls of the Eldar dead, forever shifting gender and form.
The Spear of Twilight (or Blade of Twilight) - This blade tells of Slaanesh burning out all its power to fight against something*, expending all that life essence it absorbed in the second act.
Finally, the Blade of Dawn (Dawn Blade) - this blade tells of how the Eldar, be it Ynnead or the race as a whole, will absorb the energy of its defeated foe, extending its longevity beyond what it should.
So, what is Cegorach's big joke? By helping the Ynnari form, by helping Ynnead be born, Slaanesh will realise that it must preserve the Eldar, prevent them from dying, and be doing so, starve to death, because the deaths of the Eldar will see Ynnead rise and destroy Slaanesh forever. And when Slaanesh starves to death, the Eldar will rise again, in a new dawn.
However, I have an alternative theory - Ynnead will be born early, as per Ynnari wishes. However, to become strong enough to beat Slaanesh, it must absorb the life of all Eldar, akin to a vampire, and parallel to the Ynnari using spirit stones to power themselves. With no souls left to devour, Slaanesh would starve, and so, battles Ynnead to the death, destroying them both in the process.
Now, other legends of Rhana Dandra also point to Fuegan calling the Pheonix Lords together for a final battle at RD. Now, as we know RD will change to this great Laughing God joke, whether it goes as I theorise or another way. This other way involves Fuegan being the last to die, and Baharroth dying his final death. Let's examine this for a moment.
When the Pheonix Lords come together, they are able to awaken the Avatar of Khaine without the sacrifice of an Eldar life. In many ways, this parallels the Ynnari' mission to ressurect Ynnead without sacrifice. Fuegan is said to most embody the Aspect Warriors, and believes in selective destruction rather than full oblivion. Its also notably that the other Pheonix Lord mentions is Baharoth, the Swooping Hawk. This next part requires some imagery rather than description.
The Hawk and Pheonix are both birds, obviously with the Pheonix' uniqueness being death and rebirth and fire. Say, the fire of Fuegan and death of the Hawk come together to "act" as a Pheonix, or in other words, revive the Phoenix King himself, Asuryan. Consider then, for a moment, that this would leave the combined Ynnari forces with a force related to not just 1 or 2, but 6 major Eldar Gods. Asuryan, the Pheonix, Khaine, in his Avatar form, Cegorach, who organised the event, Ynnead, Morai-Hag, from whom's hand the Croneswords are made, and Vaul who forged the Crone Swords from Morai-Hag's fingers. I am still not quite sure how this interacts with the other prophecy yet, and open to ideas
I appreciate that this might have gotten a little complicated and hard to follow, so I appreciate anyone reading through. I also appreciate thoughts and feedback and hope I've brought a new angle to light