Noctis doesn't find Prompto in Zegnautus Keep but finds Ardyn at the end of the quest instead, waiting for him. He makes Noctis an offer: let Ardyn have his way with him and Prompto can go free. Noctis of course agrees. Anything to save his best friend/lover.
Up to writer!anon if it's non or dubcon though I personally prefer noncon. Also up to writer!anon whether or not Noctis and Prompto are lovers or just really good friends.
+Ardyn taunts Noctis about the things he's done to Prompto before this and tries to show some of them by doing it to Noctis
++Prompto is watching through a monitor or one-sided mirror in another room and reacts.
+++Noctis tries to get through it by imagining Ardyn is Prompto or one of the other bros who is his lover.
Not quite sure where Gladio and Ignis are. I'll leave that to anon too.
Ardyn/Noctis nonconor dubcon to save Prompto
Up to writer!anon if it's non or dubcon though I personally prefer noncon. Also up to writer!anon whether or not Noctis and Prompto are lovers or just really good friends.
+Ardyn taunts Noctis about the things he's done to Prompto before this and tries to show some of them by doing it to Noctis
++Prompto is watching through a monitor or one-sided mirror in another room and reacts.
+++Noctis tries to get through it by imagining Ardyn is Prompto or one of the other bros who is his lover.
Not quite sure where Gladio and Ignis are. I'll leave that to anon too.