Weball the while, episode 7 cranks up the mystery and tension as zenitsu agatsuma becomes uncharacteristically serious and muzan kibutsuji himself confronts.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder hashira,.
Webeverything changes for zenitsu when he receives the tragic news of kuwajima’s passing in chapter 136 of the manga and season 4 episode 7.
Webduring the hashira training arc, a bird brings zenitsu a letter explaining what happened, and shortly after we see him being all serious.
Though if i remember correctly it's not.