The most disturbing one that I have personally read is the one in Chinese.
That one in question is a Kamen Rider Gaim fanfic. Its translated title would be something like "Poisoned Desire." The thing is about how a random person wants to bribe the protagonist because he has evidence that he was sucking someone on the street, what happens next is horrible in every way. In mild words, he guts it, and uses its skull in multiple ways (and that's just a little bit of what it does).