Usually referred as "The Incident" by Frankenstein, it is an important event which resulted in the 820 years long slumber of Cadis Etrama Di Raizel. The Incident resulted in the disappearance of Rai from Lukedonia and Frankenstein's 820 years long desperate search for his Master.
The Incident paved the foundation for the many important events of future including the rise of traitor clan leaders and the downfall of Lukedonia, the rise of the supposed human organisation "Union" and the so-called disappearance of Werewolves.
Clues[]
The Incident is first mentioned by Frankenstein in Season 1 where he says that he hid himself from the Nobles as he believed that some of them might have been involved in the disappearance of his Master. This is further cleared in Season 3 in which we see a flashback from hundreds of years ago. Frankenstein informs Gejutel that he is leaving Lukedonia as he can't trust the nobles. Frankenstein finds it suspicious that even after a long search by the clan leaders, no clue was found regarding his master's disappearance.
Frankenstein realizes that his suspicion was well founded when Raizel reveals that the traitor clan leaders created a situation in which Raizel had no choice but to use his powers. He states that he was in a similar situation as that of Seira's deceased father and Gejutel's dead son. (Seira's father and Gejutel's son went to the human world to help humans but it was a trap formed by traitors who killed them.)
In Chapter 280, we learn that traitors didn't attack Raizel directly as they would have taken serious injuries and thus be noticed by Frankenstein or The Previous Lord. Frankenstein deduces that many were involved in the Incident other than the six traitor clan leaders.
In Chapter 311, Frankenstein reveals his theory: Muzaka, the former Lord of Werewolves, was responsible for putting his Master in his long slumber. Muzaka wanted to wipe out the human race 820 years ago when his only daughter, Ashleen, was killed by humans. Raizel stood on his way and the resulting battle lead to their mutual destruction.
In Chapter 376, in another flashback, we see Frankenstein informing Ragar Kertia that he's leaving Lukedonia, expressing distrust as he felt an explosion when his Master disappeared but no traces were found by the nobles.
In Chapter 465, Maduke reveals the tragic truth about Ashleen's death: Humans and werewolves collaborated together to kill Muzaka's daughter. Noblesse: Beginning Of Destruction explains the role of traitors in it.