Okay, gotta be real, searching for Endeavor/Todoroki fics feels like walking through a minefield with most filters turned off. The dynamic is so specific and loaded—it's either intensely dark psychological horror or completely unhinged redemption arcs, and neither tends to be the flagship content on the biggest general platforms.
AO3 is obviously the first stop. You NEED the tag wrangling system for this. Search 'Endeavor Todoroki Enji/Todoroki Shouto', sort by kudos, and brace yourself. The quality varies wildly, but the tagging means you won't accidentally stumble into unrelated fluff. I found this one longfic, 'The Combustible Heart', that actually treated the abuse history with a brutal realism I haven't seen anywhere else. It wasn't romanticized at all, more like a character study of co-dependency after the war, which somehow made the eventual pairing hit harder. The comments section was a whole debate on whether the author was going too far, which was its own kind of fascinating.
Tumblr still has a weirdly dedicated niche for it, but it's more art and headcanon threads than full fics. You'll find links to Google Docs or Carrd sites from there. Honestly, some of the most disturbing and compelling takes I've read were PDFs linked from a single Tumblr post that's since been deleted. It's ephemeral, but the writers there seem less concerned with pleasing a broad audience, so they go to darker, stranger places.
I'd avoid Wattpad like the plague for this ship; the tagging is a mess, and the tone skews way too young for the material to be handled with any weight. You just end up with a lot of poorly-conceived 'bad boy' Endeavor nonsense that misses the point entirely.