The thing with Matoya and Y'shtola is the only way i can see Matoya die is for Y'shtola body possession has happened for non echo users with Genbu and Soraban, Ultima and Alma Nidhogg and Estinien but said possession has also had a high amount of aether involved being auspices a living primal or dragon eyes so if Matoya is possessing Y'shtola i think the crystal eye will be used to do it as for what sets things off i think we kill Elidibus as well as Solus releases a light aspected Black rose killing both the Alliance and Garlean main forces and this tips the balance on the source within the maelstrom of death we plead to Hydalen for a way to stop it and we get sent to the source to fetch Minifilia and bring her back but she refuses to leave the 1st to their own suffering and to combat the light there we become the WoD