When Jason goes back to the manor for the first time he calls Damian Demon Spawn. Everyone is thinking that its because Damian is Ra's grandson, but no, Jason wouldnt use facts about someone [That they cant control] to come um with piss-off-nicknames, give him his creativity back.
Jason calls Damian Demon Spawn because every time they were on a League Mission together and they would stumble upon the supernatural, Demons would ALWAYS come out from above or below Damian. He was ALWAYS in the place the demons would come from purely by COINCIDENCE. It happened so much that every time they were expecting a supernatural fight they'd turn to Damian ready to attack the demons. And it pissed Damian off every time someone would report the mission and mention in so much that Jason started to call him Demon Spawn.
Its months later when Robin, Red Robin, Red Hood and Nightwing were on a mission together and they'd knew a fight with supernatural beings were coming [It wasnt the main purpose of the mission obviously but it happens] and Jason pointed his guns at Damian that they understood. When the Demon portal came from below Damian Jason cheered SO HARD "DEMON SPAWN!!!!!!" That it made Damian try to slice his helmet off at some point











