George O. Smith 
Hellflower 

Once He had Been a Star Master-Now He Dealt in Soul-Shattering Ruin… Farradyne had committed the one unpardonable error a Space Master could make. He didn’t die along with the other 32 passengers when his ship smashed into the Bog on Venus. They broke him-exiled him to the rotting fungus fields of Venus. Now his only desire in existence was to return to the cool, gleaming sea of deep space. And there was a way-only one. But he would have to become the vilest parasite in the universe-peddler of a poison that stripped the spirit, before it consumed the body…." (Goodreads)