Work Text:
harry potter is Astolfo
astolfo's master didn't have nearly enough mana supply to hold a servant so he kidnaps an extremely mana dense child (harry) to use a base and fuel for the servant's form, it works somewhat - its more like ruler in which both harry and astolfo are aware of each other and can communicate in their mind and harry is the main supply of mana but he has more than enough
the ritual to summon astolfo sacrificed the soul shard in harry's scar leaving enough room in his body/soul/magic for astolfo to stay indefinitely without damage to harry,
they can switch whether or not astolfo's body is fronting/seen, they don't tell the masters this
fate : apocrypha goes as normal, sieg takes the grail to the reverse side of the world, astolfo stays to help people as they are technically sieg's servant, they stay mostly with ruler's ex host but still retain noble phantasms despite the grail war being finished
English wizards only find out because of tri-wizard tournemant cup failing to bind/summon the 4th champion (harry's blood, its changed just enough because of astolfo's presence to not work), the unspeakables use scrying magic to see the ending of the grail war (harry age 8) up to harry being age 14
i want to see where people can take this