Divine Judgement / Advanced Time Regression.

After they had successfully binded him, the chains froze in place. 

"Stay." Ordered John as he pulled back his arm while clenching the dark knight's neck.

In that instant, they separated.

The bright sword's body now back to normal, was thrown aside onto the floor next to john, leaving behind a single humanoid shape of a shadow standing there.

The shadow couldn't properly keep it's physique on it's own without a body, so john had maintained it for him. 

It was the shape of a demon. Upon realizing that it was able to maintain it's shape, the demon looked confused, and looked at itself in doubt.

"Huh? M-My form… It's incomplete but… It's fixed." 

The demon was confused as to why John would do this and surprised by how John could do this. Giving him a form wouldn't benefit John in anyway, in fact, it would mean the Demon would finally be able to do what it wanted, and freely too.

He thought John was an idiot. Until, he realized it.