There was a race of beings that were inhabited by another creature. They were a semi symbiotic species. You could tell when one was speaking and when the other was. Also, there was an imitator species that would trick the people into allowing it in.
The leach promised great power but only brought a slow decay. It was a slow decay of mind as well as body. The people didn’t see it for what it was so the leach inhabited just about everybody. Only those who underwent a special and sometimes painful process could be free of the leach. The process was easy and quick, but not without Sp struggle.
Our hero finds himself in the middle of this struggle. (Is he from this planet or a visitor from outside?)
Does he travel in space? Is he a pilot? Is he lost? Does he have another gateway? Is he Mark?