In this story, the Harry Potter prophecy is really about both Harry and Percy. Same old, same old.... Harry and Percy are twins, sons of Lily Evans and Poseidon. Lily and James got married when she realized she was pregnant. They both live with their muggle (and mortal) family, the Dursleys. The twins had to depend on each other (and only each other) their entire lives to survive. Everything changes suddenly as they started receiving mysterious letters... (Mainly follows the plot of The Sorcerer's/Philosopher's Stone, mostly from Harry's perspective. (3rd person POV though.)) I'm going to put this on ao3 as well at some point. Past chapters are being continually edited when I have the time to help get rid of younger me's cringe, but nothing too big will be changed without a notification about it.
13 parts