Ass Pull: Regis being able to suddenly summon the family Soul Weapon when things were looking grim in a fight against two Werewolves had a lot of people turning their heads. The closest there was to Foreshadowing was Regis in chapters prior to this event wishing for more power, because he felt that he was falling behind his friends.
Between Frank and Rai.Jake told Rai "You must be a public nemesis to all men with that pretty face." Jake either said that out of jealousy, or he was afraid that Rai was turning him gay...
As well as M-21 & M-24, & perhaps some Foe Yay with Crombel toward Frankenstein.
As for Les Yay: A rather pretty clan leader, Rozaria, stopped fighting with Seira for a moment in order to comment on how beautiful, elegant and ladylike she'd become.
Ragar Kertia sighing over the Previous Lord. Possibly also Krasis Bluster. Urokai Agvain was suspiciously Yandere over both the Previous Lord and Raizel's interest in Frankenstein.
I Knew It!: Many saw Rajak's death coming from a mile away as having the character die would mean that Rael would end up with a complete version of the Kertia Family's Soul Weapon.
Rai's "Kneel" and "I didn't give you permission to _____" have become instant memes in South Korea.
Likewise for making fun of Regis' "elegance".
Rai and Frankenstein are on the way!Explanation The second Lukedonia invasion has put a lot of focus on the Lukedonia Nobles fighting against their Traitor Noble / Werewolf invaders. And as a result, Rai and Frankenstein, who are supposed to be on their way to helping the Lukedonia Nobles out, have been sidelined. Jokes as to whether the two got lost, or are off doing other stuff along the way, have run rampant throughout the chapters surrounding the invasion.
"Dr. Arith can't handle the D." went memetic with the English speaking part of the fandom. Explanation Arith accidentally took an improved form of a pill called D that causes a huge boost in power at the cost of one's own life. "The D" is also slang for "the dick" or male genitalia.
Nightmare Fuel: The series is rather mild in nightmare, but several incidents stand out:
In Episode 74 When Shinwoo, Yuna, and Ikhan are trying to figure out which buttons in the elevator would lead them to the exit, Jake punches a hole through the door and peeks in, very reminiscent of The Shining. Then he starts prying the doors open...
When 12th Elder goes One-Winged Angel, the way his body contorts, including his face, can be a mild shock to some.
Ignes is also treated this way for similar reasons—she's a despicable person with no redeeming qualities whatsoever. Fans were overjoyed in chapter 307, when she came face-to-face with a very angry Rai.
They Wasted a Perfectly Good Character: At the start of the series, the class students, such as Shinwoo and Ik-han, looked like they were going to be established as important characters alongside Rai, but have overtime become relatively unimportant to the overall story except to remind Rai and company what they wish to protect.
Similar to what's mentioned in They Wasted a Perfectly Good Character, the abortion of Shinwoo and Ik-han's character arcs left a lot of people in confusion, and remain wondering to this very day if they're ever going to be focused on again.
During the Traitor Noble invasion on Lukedonia, Gradeus brought up an interesting question as to why Nobles must force themselves to eventually go into eternal sleep, and that there will be some Nobles, such as Gradeus, who don't wish to pointlessly kill themselves. However, this issue doesn't get delved into any deeper, and in the end, Gradeus eventually gets killed off by Franky.
Viewer Gender Confusion: Tao is a guy. The 3rd Elder fits the same category as numerous fans were shocked when it was revealed in Chapter 519 that he was male, quite humorously. Ironically, when several fans stated that there was no indication that the 3rd Elder was male, in Chapter 318, Karias referred to him as a "picky guy".
