Finding M-rated Chanbaek can be a bit of a treasure hunt, but it's totally doable. Archive of Our Own, or AO3, is usually my first stop. Their tagging system is a lifesaver – you can filter by 'Explicit' rating, the Chanbaek pairing, and then add additional tags like 'Mature' or 'Graphic Depictions Of Violence' depending on what specific themes you're after. I'd also browse the 'Dead Dove: Do Not Eat' tag if you're looking for really intense, no-holds-barred stuff, though tread carefully. Wattpad has its share, but the quality and tagging can be super hit-or-miss; you have to dig through a lot of fluff to find the darker stories.
Don't forget about Asianfanfics, either. It's a major hub for EXO fics, and while the search might be less precise, there's a huge volume of content, especially for this ship. Sometimes the real gems are on personal blogs or locked communities, which is annoying. I once found an incredible, brutal dystopian AU by following a rec list on Tumblr. It's frustrating how much good fic gets hidden away because of platform policies, but that's part of the fandom landscape, I guess.