Whatever Marianne did to Iron Star, because it certainly could not have been pleasant for her to say that "they probably wish they were dead" with the way she left them and then tells Kallen she’d be better off not knowing the details.
The vast majority of the Unistrad arc considering it's meant to be Fairy Tail meets the horror genre. To wit:
The first two chapters give amazing buildup with tension so taut, you can cut it with a knife. One reader even thanked the author for their nightmares.
Kallen's brutal shredding of Hoteye is this on several levels. For one, it shows how powerful Kallen is that her demon form was able to take down a member of Oracion Seis, all of whom were given a power boost in this fic. Second, it speaks of Nonette's ability as she is, insofar, the only character to actually put Kallen down when she's going ballistic. Thirdly, the shredding itself is terrifying. Kallen could've easily committed murder, like the first time her Superpowered Evil Side got out.
Kalina's magic in general is a bit of this if you have a vivid enough imagination. The things she summon sound like they were designed by H.P. Lovecraft.
Our first sneak preview at Zero has him taking down the members of Oracion Seis in record time in a flashback when they tried to rebel against him.
Kalina's ultimate nightmare, Hypnox. First off, it's half the size of Domus Flau and it took down Miala and Nippy, who are capable of handling Devil Slayers. Also, this is her strongest nightmare and a significant weaker one was able to put down all of Team Quatro Cerberus without much effort. Think about that for a minute.
It's gotten worse. Hypnox was able to take on Tianzi and Nivea at the same time and win.
Nirix. Much like Sindel, Nirix is truly a worthy Arc Villain as he has begun his rampage by taking out Kallen, Rogue, and Sting in record time.
Mordred. Everything about the guy is outright terrifying. He's Lelouch all out of fucks to give and shows exactly what happens when Lelouch loses all sense of morals and just does what he feels is necessary for the greater good.