My Thoughts on Jason's origin:
One of the enduring questions of the Friday the 13th saga is where did Jason come from and how did he become nearly invincible. Like many fans I have my own theory on the subject.
Back in 1958 Jason did drown Crystal Lake; his body did reside somewhere at the bottom of the lake, it was just never recovered.
After he died his spirit was tormented and it began talking to his mother. She was not just an crazy woman who thought her dead son was telling her to kill those kids at the lake. She was a mentally unstable
woman who was driven insane by her dead son's spirit. When Mrs. Voorhees was imitating Jason's voice in part 1 she was really hearing Jason talk to her in her head. After Mrs. Voorhees was decapitated with a
machete Jason's spirit was even more tormented than before. When Alice fell asleep in the canoe Jason's spirit was influencing her dreams, just like Renne in Part VIII who kept seeing visions of Jason. Alice was not really
pulled out of the canoe, when it happened in the dream it scarred her so much that she bolted awake and fell in the water. His spirit was also what Rennie had seen when she thought she was pulled under water by Jason.
With Mrs. Voorhees dead Jason has no one to do his killing for him. Somehow (I'll get to how later) one of those slug creatures from Jason Goes to Hell craws out of his body. That slug thing goes to Mrs.
Voorhees' body and is reborn; remember that he can be reborn through a Voorhees whether she is alive or dead. Jason was not reborn as just an ordinary man, he was reborn as a man with superhuman strengths. He has
incredible strength and the ability to regenerate. Every time the body is impaled or sliced in the head with an ax it just takes some time for it to regenerate. The body does have some limits, it requires food (Jason had snare traps set up to catch animals in part 2. Even though the body is strong it is not invincible; being cut up enough will cause it to die, as happened in part IV.
After Jason died at the hands of Tommy Jarvis his soul was no doubt tormented again. Unlike when he was a child there was no one who he could push into carrying out his vengeance. It's possible that his spirit helped push the paramedic over the edge in part V, just like he had pushed his mom over the edge all those years ago. It is also possible that Jason's spirit was influencing Tommy Jarvis and it planted the idea of digging Jason up in Tommy's head.
Since the body is not quite human an electrical charge can bring it back to life even years after it's death, as happened in Part VI. After part VI we discover another limit that the body has, it needs Oxygen; after being chained to the bottom of the lake at the end of part VI it needed Tina's telepathic powers to revive it. After part VII Jason was pulled under water by Mr. Shepard, who Tina temporarily revived by with her telekinetic power. Presumably they both died underwater without oxygen. At the beginning of part VIII a powerful electric charge brings the body back to life, just like the lightning strike in part VI. At the end of part VIII the body is pretty badly damaged by toxic waste, but not so much that it can't regenerate. As for why he looked like a man agian, that was a hallucination; we were seeing it thought the eyes of a girl who has been seeing things for the entire movie.
In Jason Goes to Hell the FBI destroys Jason's body more than anyone in the previous films ever did. It was damaged beyond any chance of ever regenerating or being revived. The spirit messed with the coroner's mind enough to make him eat Jason's heart. After eating the heart the man became Jason. Jason can only be reborn as a man through a Voorhees so all that happens when the man eats Jason's heart is that slug creature is reborn inside the man. Until it finds a Voorhees woman all that the slug can do is posses other bodies for a short period of
time.
In Jason Goes to Hell Creighton Duke knows a lot more about Jason's origin than anybody else. Presumably Duke was into the occult; when he studied the Jason stories he recognized them as some supernatural phenomena that is perhaps written in some ancient text. Perhaps some evil demon can help tormented and twisted souls seek their revenge. After doing some research he found out that Jason could be reborn through a close relative. He also saw that the only way to kill him was for a close relative to stab his heart with that magic dagger. After he
was stabbed by a Voorhees with the magic dagger he was taken, body and soul, to hell.