Hi! So basically yeah those were Kaentake's spores that he inhaled. The Kaentake (aka fire coral mushroom or Podostroma cornu-damae) is the second most deadly mushroom in the world and ingestion causes symptoms of organ failure, skin peeling, and brain shrinking which I think(?) happens quickly in the world of the manga given how the spores cause mushrooms to grow instantly. Plus I imagine considering his fire powers, Kaentake's spores probably feel like they're burning too. Pretty rough way to goWhile I dont like unnecessary torture but the mastermind got off easy, though I'm not sure what did he make him inhale, his own spores? Or little fire spores that burns the lung alveoli cell thing? If so then shit gotta be torturous anyway.