The traitor nobles are the six clan leaders who betrayed The Lord 500 years ago after The Previous Lord entered his eternal sleep along with the other clan leaders. They frequently visited the Noblesse in his mansion; the other clan leaders refrain from visiting him unless it is of utmost importance because they know that the Noblesse favored less company. It is revealed that all the traitors were once the closest followers of Cadis Etrama Di Raizel, although they betrayed him 820 years ago which caused him to enter his long slumber. Not only did they conspire to have him killed, they also used him during their betrayal. This is why Raskreia believed that Raizel was the one who leads the traitor nobles. Around 100 years ago, they set a trap to lure out nobles by placing some humans in danger. This trap resulted in the death of Regis' father, Rousare, and also the death of Seira's father, the former head of the Loyard Clan.
Additionally, Ignes Kravei is also considered as a traitor since she violated the Nobles' laws in a quest for power.
Three of the six traitor clan leaders are elders of the Union. Roctis Kravei, Urokai Agvain and Zarga Siriana were the 4th, 6th and 7th elders of the Union respectively. The other three were not part of the Union; however, they are associated with the werewolves. Although all six of them are already in eternal sleep, only four of them were executed by the Noblesse.
Even though they betrayed Raizel for their own reasons and attempted on taking his life, their reverence for him did not disappear; in fact, they still address him formally and mentions his name with utmost respect.
Here are the 6 traitors' reasons for their treacheries:
Edian Drosia: Lagus used Edian's guilt and feelings for Raizel to his advantage and used drugs on her to turn her into a puppet. She was always in a dream-like state and never realized that she was becoming a tool to put Raizel into Eternal Sleep. This resulted in her strange behavior during the Lukedonia invasion. She broke free from the puppet-like state and sacrificed herself to protect Raizel from Lagus' finishing blow.
Gradeus: Dissatisfied at the Nobles' code of conduct and their ways of life which restricts him from wielding his powers however he wants, Gradeus sees this as his inalienable right. He was killed by Frankenstein and his soul was absorbed by the Dark Spear.
Lagus Tradio: To usurp the current lord of Lukedonia and become the lord himself. Then he would set out to reign over all walks of life. He wants nobles to rule over the world. He was executed by the Noblesse.
Roctis Kravei: To protect his own but felonious daughter Ignes Kravei, who violated Nobles' laws. He was executed by the Noblesse.
Urokai Agvain: Had a bitter feeling of betrayal towards Cadis Etrama Di Raizel who chose Frankenstein over him, and Jealous of Frankenstein who 'stole' his master's heart. He betrayed him because he felt that Raizel gives humans more importance than the members of his own race. (As Noblesse, Raizel will execute his fellow nobles if they will harm humans). He was executed by the Noblesse.
Zarga Siriana: Incompatible and irreconcilable difference of values/principles with Lukedonia. He was executed by the Noblesse.