James Potter, from several generations of inbreeding, is actually infertile. Merlin who has known of Arthur’s resurrection but hasn’t been able to properly locate him due to some other ancient magical influence, has been sending him support through their connection, until he can find him. Decades go by, and by the time he does finally find him, Voldemort is at the height of his power. Merlin seeing a vision of the prophecy, and understanding its intended meaning, transmutes his form through pure magic into James and Lily Potter to be born as the boy who lived, Harry Potter. Neither Merlin or Arther remember their previous incarnations, but are further bound, by murder, magic, and love.
Optionally, the majority of the series plays out as typical aside from the writers choosing. Harry and Voldemort are their own identities just with the added layer of “we’ve met before”, so like, EVEN more reasons for Voldemort to obsess over Harry besides his horcrux and all their similarities. Harry just really wants to save Voldemort from himself, and has this unbearable yearning. Moral alignments are also optional, although I personally chose Voldemort to be Arthur’s reincarnation because (his ego is king sized) he wants to “unite” and “protect” wixen, plus Arthur’s distrust of magic has become his hatred of muggles. Merlin (Harry) still has to set our dollophead straight, smh. Also optional is Voldemort’s appearance. Personally love beautiful monster snake (I accidentally typed snack here) Voldemort, but demonic charmer Tom Riddle face is always welcome ;)
All other characters are however the writer wishes to write them, although I sort of imagine no one is aware of who Harry or Voldemort were, accept eventually, each other. Emrys began hiding centuries ago so nobody even knows about Merlin other than lore. Y’know and his saggy balls.
As for the ancient power that kept Merlin from Arthur, this is fully up to writer! Could be nothing, could be totally a major plot point.
Any rating and content is welcome
thank you for reading my prompt (▰˘◡˘▰)
bonus points if Harry eventually comes to respect Voldemort’s identity he made for himself and stops deadnaming him as “Tom”. (Just genuinely my favourite trope)
edit: oh my god, this is why they both feel at home in the castle ughhh my heart (╥_╥)