AND THEN WE SAW IT!!!
http://imgur.com/TChYBA0
OH.MY.GOD. Do you see it?! DO YOU SEE IT?!?!?!?! It's been in front of our faces this whole time!! We were all as blind as Ignis!
Ifrit burst out of Taelpar Crag, carrying Eos...and lit a Pyre at the nearby Rock of Ravatogh. He laid his beloved Eos down, lit the Pyre, and collapsed in death next to her, and while embracing her body, burned along with her. As her corpse burned, the ash and smoke rose in the sky, releasing the star scourge throughout the planet. And all that remains are her fossilized wing, and his crystalized horns, still visible from atop The Rock of Ravatogh to this day.
http://imgur.com/829cqW6
During the quest ”Ascend The Rock Of Ravatogh" Vyv says, ”if you really want to impress people, you've got to show them something they've never seen before. They say there's a cave near the summit that offers the finest view in all of creation." THIS IS A SUBTLE HINT FROM SQUARE ENIX! When you arrive at the cave to photograph it for Vyv....there's an item nearby. A RUBY BRACELET. Now why say it's the finest view in all of creation, when it's just a tiny cave with lava that you can't even enter? Let's take a closer look at it....
http://imgur.com/hxgqEhd
Huh. This reminds us of something...
http://imgur.com/4knAvLR
OH.MY.SWEET.BILBO.BAGGINS. IT'S EOS' HEART! It has a hole in it...the same shape and size of the Dark Lucian Crystal. Let's see what Bahamut has to say about the Crystal.
http://imgur.com/XF86yoy
The soul of the star is in the crystal. The sun is a star. Eos' soul is in the crystal. This is why it can restore the Dawn. The crystal contains a small shred of the goodness and light that once was Eos. Ifrit saved her just in time, before the last of her light was extinguished. If he had not gotten Eos' out when he did, and purified her body with the Pyre, her light would've gone out, bringing eternal night. Burning her on the Pyre spread the star scourge into the air, but it was a necessary evil.
But how was the Crystal retrieved?? Let's look at Ifrit during the Citadel battle.
http://imgur.com/PpxwtZR
After Ifrit transferred his soul to his nearby messenger, he must have returned to the Pyre after the flames died down. He must've cut open her heart and reached inside to retrieve the Crystal. That is why he has burns on half of his body. Her burning, crystalized heart was still that hot! At some point, Bahamut must've stolen the Crystal from him. Also, at some point after his first death and retrieving the crystal from Eos' heart, Ifrit in his messenger's body must have built Pitioss ruins.
At the cost of his life, at the cost of becoming a Betrayer, at the cost of spreading star scourge unto the world. IF IFRIT HAD NOT SAVED EOS WHEN HE DID, THE LAST OF HER LIGHT WOULD'VE EXTINGUISHED, AND THERE WOULD NEVER BE ANOTHER DAWN!!