Since I don't have access to the specific 'the phantom killer true story', it's difficult to determine precisely who the phantom killer was. It could be a local criminal who gained a reputation for being almost invisible when committing murders or other heinous acts. There could be a whole backstory about this person's upbringing, relationships, and how they developed their modus operandi. Maybe they had some special skills or knowledge that allowed them to be so elusive, like knowledge of the local area's back alleys or a talent for disguising themselves. But again, this is all speculation without more details about the story.
It could be a lost or abandoned submarine. Maybe it was part of a military experiment that went wrong, and it's now drifting around, appearing and disappearing like a phantom.
Well, it depends on the specific story. Some 'phantom submarine' tales might be based on real submarines that were involved in covert operations. For example, during the Cold War, both sides had submarines doing secretive patrols and surveillance. If one side detected an enemy sub briefly and then lost track of it, it could be called a 'phantom submarine'. So in the context of a true story, it might be a real military sub.