Now if I may allow me to analyse it with a bit of speculation. So basically after Origins when Richtofen separates from the other 3, he came into contact with Monty (somehow, I assume we'll find out, maybe in the comics or more wisps?). Monty, being omnipotent and more powerful than Richtofen, then sends him on a mission to retrieve the Kronorium and therefore the Summoning Key, and then Richtofen returns to the other 3 and begins the quest of retrieving their souls.