How Harry Became Merlin

From archive of our own

The description: There's an old, powerful magic in the air at Hogwarts this year, and Harry is hypersensitive to it. The problem is he can't figure out why. When the Charms award starts showing him visions and a strangely familiar voice starts speaking to him telepathically, Harry is determined to get to the bottom of it. An old magic book is revealed, and Harry finds himself pulled further down the rabbit hole than he'd ever imagined, causing his understanding of himself and his magic to be shattered forever.

Reincarnation AU for Harry Potter: GOF.
Canon divergence for Season 5 of Merlin.