This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Descendants of Darkness
The scene featuring this phrase: If you don't have anything to live for... at least live for me! as Hisoka desperately tries to convince Tsuzuki to not commit suicide after muraki mind rapes him completely. Oh, Hisoka. In the episode in question, it became apparent that the two Yaoi Guys were getting a happy ending.
All of Tsuzuki's flashbacks to his childhood. A little boy being stoned because of his eye color? Wibble!
Hisoka's past and the events leading up to his death. This guy has. had. it. BAD. And all of these aforementioned tropes occurred before he was even seventeen years old.
Not to mention if he hadn't encountered Muraki that fateful night, he would have still been doomed to be raped. Apparently a demon is getting passed from generation to generation in the Kurosaki lineage, inhabiting the bodies of every clan head and torturing them every night. Hisoka was next in line.
Hisoka's breakdown after he kills Tsubaki, a girl whom he had befriended, to end her suffering. Usually a stoic teenage boy, this horrendous and heartbreaking act leaves him sobbing hysterically and wondering if he'll turn into a monster like Muraki. Tsuzuki hugs him and reassures him over and over again that he's not a monster.