Events that shifted the story Theme and Genre by turning it darker and serious

Jujutsu kaisen: "Yuji reacting to mahito and sukuna laughter and realising that his story isn't a run of the mill shonen but a seinen disguised as a shonen "

Harry potter: "Cedric Diggory death caused the Harry potter series to be a dark adult urban fantasy with heavy themes of death betrayal and prejudice"

Invincible: "Omni man killing the Guardians is the turning point of Invincible being a mature existential show of aliens and powers"