The Emperor was an angel of esus, not archangel.
Korrina is Gyra's daughter, and therefore immortal... No, it hasn't happened in any other lore but that doesn't mean impossible, only that males are more easily tempted by mortals.
Esus sent Rahserat to Arawn's vault in order to weaken Arawn, and open his vault to Hell, in order to 'save' the world from the One... I think that fits his lore.
While in Arawn's vault, Rahserat persuaded souls to join Laroth, always believing he was doing what was best for Erebus... He disliked Laroth however, and came to doubt that he was any better... His doubts manifested themselves in Arawn's realm as a false Laroth. This imposter was the one the majority of the souls Rahserat influenced went to, and he obeyed Rahserat's commands completely... This is the source of the Scions.
There is mention of a 'Hidden Queen'... Thinking this is Ceridwen. She aids Rahserat in his entrance to Arawn's realm, in return for information Esus promised her. Rahserat believed he delivered the info... Unfortunately Esus had decieved her, and his message was empty.
He left the vault early, after realizing that the False Laroth will be able to finish the job easily.... The Hidden Queen catches him as he leaves, and traps him in a place where all is one. I'm thinking this is a dimension Ceridwen created in order to kill the One... When everything is the same there can ultimately be no creation. This dimension is the Vault, from which awakened come... Over the centuries he was trapped there the False Laroth kept sending him souls.
Korrina had been sent ahead by Gyra, to open Rahserat's path back to Erebus. Gyra told him 'She knows who he is', which I believe means she sees his potential to fall from Esus's service, becoming the Angel of Regret (Get some angels in the alignment event). She had asked him to 'foster' Korrina, so she can experience life. After centuries of waiting, of testing the door on the same date each year, the door finally opened... Onto the Vault he had been trapped in. He was prevented from leaving through any existing path, but over time Korrina managed to forge a new one for him.