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Kazuya had feigned losing consciousness after getting rid of Dokuro Domon. His mana was almost depleted because of the miscalculation of the mana required to store all the things in Domon's panic room so he pretended to lose consciousness to avoid any questions they had. He truly was tired and didn't want to involve himself in their useless banter.
Like a log, he was able to hear them talk and with his senses, he quickly discovered that Domyouji Kuuga left secretly. Kazuya was able to have some time to think about what happened and what he did to someone he had considered one of the few people who lent a helping hand after he had lost everything.