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,. 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. Webdemon slayer chapter 144 confirms what was said in zenitsu’s letter as he speaks to kaigaku in a heated discussion. The message indicates that jigoro kuwajima,.
