The Obsidian Chamber

After the apparent death of Agent Pendergast in the previous novel, Constance Greene has returned to Riverside Drive, but Diogenes reappears and seems to kidnap Constance. Proctor pursues them by plane to Canada and Africa, and by car into the desert where he is stranded. As it turns out, the chase was merely a diversion. Constance is still grieving in the basement of Riverside Drive and is slowly seduced by Diogenes, while the surviving Agent Pendergast has been fished out of the sea by smugglers who want to use him to free a colleague from prison and plan to kill him afterwards.

Pendergast manages to free himself, and just as he is about to return, Constance gives in to Diogenes and follows him to his island. Diogenes tells her that the incident on the volcano changed him and he is now working on two things: reconstructing the potion that gave Constance such a long life, and making her happy. He even tells her he gave up killing, though in truth he seems to have no such inhibitions, as he is perfectly happy to kill some people in order to gain the last ingredient to the potion.