The lore drop here is phenomenal, it is really interesting for me what actually happened to Sienna.
So far into the story, I think Vermouth formed some kind of a deal with the demon king of incarceration to reincarnate Hamel 300 years into the future. As for the reason, it could be that Vermouth knew that he would be doomed if the fight continued with Helmuth. I think Vermouth knew the hidden potential and true talents of Hamel, and that the only thing stopping Hamel from becoming stronger than him is his genes and body limitation. That is why he asked the demon king of incarceration to reincarnate Hamel as one of his descendants. Of course the other side of the deal was to leave the two left demon kings alone and never oppose them again.
