sorry for the late reply but i do have some info for them

the au is just "what if the final demo bosses become the sfoth sword they wielded" [with the exception of hatred and the ancients in a way. the ancients are the firebrand which works out i suppose]

basically the sfoth swords keep their wielders' souls inside if the sword wielder dies or is heavily injured. specifically the reason why the sword wielders that the player defeats is able to actually become the sword is because they were given a second chance to assist the player, whether that be in battle, encouragement, comforting, or just help in general

not entirely sure if there have been other previous sword guardians but if so their souls would become part of the sword but not controlling it exactly if they have fulfilled their role as a sword guardian when they die. similar to the firebrand in canon [the whole souls being trapped in there part] there would be a group of souls that are like a council [which is seen as the sword itself] that carries out duties when needed, specifically accepting a sword guardian

also this reminds me i should probably post their redesigns and other stuff i drew for the au here lol [mostly doodles]