We saved Auriel from the Lord of Despair, it was him who captured her. Imperius was about to attack us if Diablo didn't get to the spire when he did and gimp the angels. Meaning he's already heading down the path to wrath. Had they all died if we didnt save them fate would have been alone and shattered.
The thing is though it was a fake out prophecy. We were meant to think it was happening during Diablo 3 but all we were doing was Mathael's work for him. We are Mathael's number one minion and we don't even know it. I mean how much stuff do we kill for the lord of death? The prophecy is really still about to happen as Mathael goes around and puts the angels in the black soulstone. Fate will be the last one to go.
I have a feeling he will make his first appearance when we battle Imperius during the angel civil war that is coming and possibly kill Tyreal. As we kill Imperius Matheal will appear to us, and reveal that he was Adria all along playing diablo and us like a fiddle, and steal Imperius and Tyreal's soul thanking us for doing his work for him. He will then head off to get the weaker angels himself. The rest of the game is us chasing him in typical diablo format.
He was the one who broke into Kuules tomb before we even got there and but meph's and baals, and the lesser evils from D2 in the stone. He's been playing the heros, andgels and demons since D1. Kuule was even warning us about him, I believe he knew Adira wasn't what she appeared.
Leia is also still alive I believe as she is part angel and demon and her soul could be contained in the soulstone, and will be communicating with us somehow. She will eventually serve as the bridge in the stone to balance the angels with the demons and keep them all contained. The stone will no longer have only a certain timeframe to hold souls this way. Matheal will be able to use it for whatever he plans then but will receive resistance to by us time from Leia.
I called this all months go in some threads as did others. Expansion definitely seems to be heading down the predicted path, for once. Usually were all way off lol. The black soulstone was definitely going to be used for angels at some point, why else would they tell us it could be used for that, yet not put a single angel in it in D3? Mathaels goal is to gain the powers of Anu and Tathamet(through the black soulstone) to become a god and reshape the universe as he sees fit.