Wow, this is a super niche corner of the fandom, but it rings a bell! I remember stumbling across a story years ago that had this wild premise where Harry wasn't just the 'Boy Who Lived' but was an actual cosmic entity, like a personification of entropy or something. It wasn't on AO3, I think it was on one of those older, clunky independent archives. The writing was incredibly angsty and philosophical, focusing less on magic duels and more on Harry's internal horror as he realized his very presence caused decay and catastrophe. He'd try to save someone, and his touch would just... unravel them. It was profoundly bleak, almost like a Greek tragedy set in Hogwarts.
I don't recall the title, which is maddening because the concept has stuck with me. The author really dug into the isolation of it—here's this kid who's supposed to be a savior, but his fundamental nature is to destroy. It made the usual 'grey Harry' or 'dark Harry' tropes look tame. The conflict came from him trying to build connections while knowing he was a walking apocalypse, and the narrative wrestled with whether 'good' and 'evil' even apply to a force of nature. I think it might have been a crossover in spirit with something like 'Sandman', though not officially tagged as one. I'd kill to find it again, honestly; it was more of a character study than a plot-driven adventure, and it handled the godhood thing with a disturbing amount of gravity.