the landscape is pretty fragmented these days. Archive of Our Own, AO3, is still the main hub because its tagging and filtering system is unparalleled. You can filter by the 'Explicit' rating and then add additional tags like 'Dark Harry' or 'Adult Themes' to narrow things down. The quality varies wildly, though—some 'Explicit' fics are just smut with a plot so thin it's transparent, while others have genuinely complex, adult-level political or psychological arcs. I'd steer clear of FanFiction.net for this specifically; their content guidelines are stricter, and a lot of mature stuff gets purged or hidden in anonymous collections, making it hard to find reliably.
Sometimes, dedicated communities on sites like QuestionableQuesting or SpaceBattles' 'NSFW Creative Writing' subforum host more niche, novel-length works that explore darker or more mature concepts beyond just sexual content. These forums often have a higher barrier for entry in terms of writing quality and reader engagement. For specific pairings, Discord servers are goldmines; you often need an invite from someone already in the community, and they'll have channels dedicated to recs and links you won't find easily through Google. Just be prepared to wade through some truly bizarre stuff—the freedom to write anything means people really do write anything. My bookmark folder is a testament to both brilliant finds and things I wish I could un-read.