Noctis and Ardyn were lovers in the past when Ardyn was king/healer. When Ardyn was betrayed Noctis was killed/died in the process. (Maybe this is why Ardyn turns dark)? But when Noctis is reborn Ardyn recognizes him and, despite having to fight against the scourge inside him, Ardyn sincerely tries to help Noctis and get him to remember their past. Cue a happier (or even angstier it's your choice) turn of events than in the game.