I'm asking if any of you are able to remember the name and year of an old series (from the seventies?) about an immortal guy that was permanently hunted by authorities who intended to perform experiments on him to find out how to become immortal (if I'm not very wrong). What I'm sure about is that those hunters used to shoot the immortal man with tranquilizers in order to not kill him, as he had to stay alive to be able to deliver clues about immortality.
Probably The Immortal from 1970.
Ben Richards is a test car driver for a large corporation owned by billionaire Jordan Braddock. He's 42 years old, but looks young enough to pass for 25—and he's never been sick a day in his life. Ben's life changes when he donates a pint of blood. When Braddock, who is dying, is given a transfusion of his donated blood, and is brought back from the brink of death, Ben's physician, Dr. Matthew Pearce, determines that his O-negative blood contains all known antibodies and immunities. This gives Ben immunity to every known disease and an estimated lifespan five to ten times that of other humans, making him, in the doctor's words, "virtually immortal". The billionaire decides that he has to control Richards' life so he can access his life-saving blood.
This mention of the series in Science Fiction Television Series: Episode Guides, Histories, and Casts and Credits for 62 Prime-Time Shows, 1959 through 1989, Volume 1 states that "[Ben] was susceptible to the tranquilizer darts used by Fletcher's men"
Possibly the Casca books. The first two were written by Barry Sadler and the rest simply kept his name on the cover while actually written by others. The series takes its name from the hero of the series, Casca Rufio Longinus. The first book tells his origin as a simple Roman soldier who stabs a certain someone who was crucified in the side. That person looks down at him and says "Soldier, you are content as you are, and so you shall remain until we meet again. "
He finds himself with what I can only describe as a Wolverine-level healing factor. He literally can not die. The book series follows him around the world over the next two thousand years, generally as a soldier.
We find out a couple of books in (I do not remember exactly when) that there is a cult who have been tracking him since the beginning as best they can. See, they know about the prophecy. The way they figure it, this man was personally promised that he would meet their messiah upon the messiahs return, so these people follow Casca around trying to find their messiah, among other things so that they will know when doomsday occurs.
As memory serves, Casca first finds out about them when they knock him out and kidnap him, and he observes one of their rituals where they reenact his stabbing on the cross. Casca is baffled to realize that they have his actual spear, he recognizes it. I think he ends up getting his hand cut off while escaping, but it ends up okay.
The cult return occasionally throughout the series, but they are not the focus generally.