Because They Had Hope (chapter 47 of 49, 6683 words)
Auron tries to gather his spirits and continues his journey, struggling through the Gagazet trials and eventually arriving in Zanarkand, where the visions of previous summoners show him the true meaning of the Final Summoning. Enraged, he attempts to confront Yunalesca.
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I'm not sure what the precise interaction between magic and science is in Spira such that Auron has some grasp of the concept of certain liquids being antiseptic … er we'll let that one slide.
Then he sighed, raised his head again, and went back to cleaning his wounds, wondering if this was how it would be forever
No, it's ok! It's going to get (checks notes) worse!!
The closest thing I have to an in-world explanation for the Gagazet trials needing to be redone at this point is that maybe they reset themselves once a Calm starts, somehow? The real explanation is that I needed an excuse to make Auron have to go for a wee swim; I thought it was fitting to have him needing to go into the water in these last moments, as a way of forcing him to confront his fear and close the chapter of his life where it was a central concern.
I suppose the Trials need to be reset more often than at the start of each Calm, actually, if some summoners make it to Zanarkand but then don't go through with the Final Summoning. Perhaps each time a summoner gets to Zanarkand, although that doesn't explain why Yuna and her guardians only have to do them once. Maybe it's once the Calm starts, plus certain other arcane conditions. After Auron's through with the second one it'll need to be reconstructed more substantially as well, surely in a way that requires human intervention. The maesters undoubtedly have a secret taskforce with some responsibility for these, although I don't think there's been any time for human interference since Braska's Final Summoning.
Swimming with a big sword strapped to one's back is surely not very possible, but we're following the Spiran school of physics where Tidus can clearly slash his own sword about underwater with no difficulty.
I find it odd how Yuna and her guardians see the vision of Yocun's guardian saying she'll gladly give her life and don't think, hey, maybe guardians might need to be sacrificed in some way here. Auron is a bit more intelligent than the rest of them are, I think. Also it's interesting how Yocun and her guardian seem to know the truth before meeting Yunalesca, although it seems as if they've perhaps only just found out. My headcanon is that hundreds of years ago it was fairly widely known, but Yevon succeeded in covering it up, so by Yocun's time it only (somehow?) became apparent as they got close to Zanarkand, while by Braska's time it remains a complete secret until the summoner actually speaks to Yunalesca.
"We can hide from them; we can live our lives, I promise."
They're lesbians, Harold.
… And also Gandof was gay. As Kinoc says in chapter 30: "Did you ever hear the rumours about Lord Gandof?" It's another thing that Yevon has covered up, very successfully on this occasion. Gandof is adamant that his guardian's name will be remembered, but with the loss of the knowledge that a guardian has to become a fayth, their names have also been lost, as there's no longer any understanding of their significance. (Apart from Zaon, I guess, but I don't think it's widely understood that his role in the first Final Summoning was analogous to the guardian's role in subsequent ones.)
So Auron finds out three things pretty much simultaneously: Yevon has been concealing the sexuality of one of their most vaunted figures; Jecht will become Sin; an eternal Calm is impossible under the current system. That's probably sufficient grounds for completely losing it.
The fragments of multicoloured stone are the stone that formed when Jecht became a fayth; by smashing it, Yunalesca has ensured that his consciousness is completely transferred to the aeon, and therefore to Sin. The other fayth have their stones available to inhabit most of the time, and just lend images of themselves to the aeons. This lore is entirely fabricated on my part, but it's an attempt at explaining why, in Auron's words, Sin is Jecht, while other fayth seem to remain separate from their aeons (hence some of them being able to manifest multiple aeons belonging to different summoners, as we see in the Belgemine and Isaaru battles).
Just Yunalesca casually saying "your Braska" to Auron, yikes!!
The canonical "because they had hope" line was the reason why I had Braska and Auron repeat the word to each other so much; that allows it to gain a particular significance when Yunalesca says it, and to become, of course, the final trigger for Auron's complete loss of control. It's quite the journey he's been on.