imagine an au, where dream thinks he destroyed Nightmare finally. But nightmare is not a physical creature. It used to be a light when it was passive but passive and nightmare are not the same person either so Nightmare is just a goo of negativity.
Think about a scenario, where Dream thinks he destroyed Nightmare finally but still has suspensions about it. But ink makes sure dream is thinking that Nightmare is gone. But that little piece of goo that Dream didn't realise was actually with Killer. And he possesses over Killer's body without even killing him like he did with Passive. He just makes Killer suffer because of the burning feeling of negativity but he can't die as well because Nightmare made sure Killer's soul doesn't take Nightmares presence as a threat to Killer's body. So killer just feels like its harming him, in reality nothing happens to him physically.
I don't have any idea if it makes sense. But if this idea wasn't taken before I hope no one else takes it.
Edit: i wrote this at 3 am. This sounds so stupid.