The Slithery Child of The Crescent Moon

(Next Gen AU) "Koro-sensei's legacy has been passed, but not in a way the Class E has been expecting." Seven years have passed since Koro-sensei had faded away in the world, but the now adult Nagisa, Kayano and the E-Class are in a surprise when Karasuma called them to come back to their former assassination classroom for a mission: to handle the young son of Koro-sensei and Aguri.