Although Kai completely destroyed Thorax's body, he made sure to keep his soul intact. Technically, Thorax was dead, but because Kai held on to his soul, he didn't enter the reincarnation cycle. So instead of being dead, Thorax was actually in limbo.
"W-What are you doing?! We needed information out of him, not him dead!" The king nearly lost his mind. Hero or not, he would not allow for this, especially when his family's lives are on the line.
"Relax, I know what I'm doing. Simply prying information out of his mind isn't going to work. We need to break him first." Kai could have technically read his mind, but he didn't want to since this method was more fun.
Slowly, Thorax's body started reconstructing itself, blood, guts, and body parts moving in reverse as if time had reversed. Eventually, Thorax's body had been fully reconstructed as if he had never been obliterated in the first place.
