Captain Rigg was the commanding officer of the Empress when it collided with the Hecate upon emerging from hyperspace above Azure. He originally helped the Fourth Doctor in his investigations, but Tryst accidentally spiked Rigg's drink with vraxoin, having intended it to be drank by Romana. This caused Rigg to become an addict.
When the drug began to ware off, he attacked Romana, demanding more drugs, but was shot and killed by Fisk. (TV: Nightmare of Eden)