Astarion got his hands on an artifact—some medallion with an engraving of a fish—that allows him to travel to other universes for significant magickal cost.

Because of his ties to it, it's conveniently less taxing to travel to his home world and therefore easier to go to the location of reincarnates of those from his home world than ones with no familiar souls.

That's precisely what happened when he came upon this world. He was in close proximity to reincarnate. But who were they?