Aurene's vision in Ls4 doesn't just shake Leo for the obvious reasons, in fact, nobody knows exactly what he knows: he had taught Aurene revival necromancy against his own morals and beliefs, way back in tarir when she was young. Because despite how much he preaches about immortality being a sin, or defying the natural cycle of life being abhorrent, he could not stand the thought of his daughter dying. What gets him, I think, is watching her die in battle. In war.
Leo is a Charr who is so staunchly anti-war half his purpose for life is undoing the affects of King Adelbern's control over Ascalon due to his inmense empathy for it's ghosts, and the other to change legion society so that his people would be free of the chains that bind them to endless battle and bloodshed. This is specifically because he watched too many charr die needless deaths on the frontlines, where corpses became numbers, and centurions would consider a temporary victory over ghosts as something worth celebrating. He is a charr who never knew his parents, and was never expected to be gentle, put in a position where he would need to become one. The entire world depended on his ability to be a good father to a dragon. I think he enjoyed every aspect of raising aurene Except for the battling part. For every small victory in teaching her how to run and play and hunt, he was reminded that her ultimate purpose was to be a weapon of war against her kin. He hated it, he despised it. And then he had to watch her die again and again and again in m battle. In his love for her he forgot her true purpose, the ultimate goal, and it sickens him to his core that her destiny would be to die before she even truly got the chance to live, because she is needed in the war against kralkatorrik.
And then theres the Other thing, Leo may be Aurene's champion, but he was always a supporting role in the Commander's story. Yes he finds the confidence and drive to really the initiative to come into his own post The Departing, but it all feels Different when Destiny and Prophecy come into question. I don't think imposter syndrome is the right wording, but I think its just....overwhelming in a sense he cant comprehend that HE is this destined figure. He wasnt the pact commander leading forces against elder dragons, he didnt hold the power of a god-touched human relic, he got to his position honestly through a chance meeting that bolstered his ability to do things his way. He doesnt feel...special, because in the machine of the charr high legions, he was always an anomaly. Maybe anomaly IS the right word. Leo has never felt like the life he currently leads (a tribune of the blood legions, a soldier) is the one he was born to do. He has always felt outcasted be it by his own people, his magic, his ideals, his beliefs, his very purpose. And now suddenly, they want Him. Not the commander, HIM, to lead Aurene into Kralkatorrik?
The "Vlast failed because he didn't have a champion, he didn't have You." Is a hell of a line to say to Leo, who has been in the background for so long and who has been used as a supporting role to the commander, or a pawn for Bangar to place whenever and wherever he would be needed. He does kind of dissociate and goes off to just Think for a second about everything. Because god. Can you even imagine.