I read this book some time in the 1990s. It was set during the near future. The plot revolved around the discovery of a species of humans who had established a civilization underground, but they were in decline. They occasionally made raids where they would capture normal humans from the surface for enslavement, and sometimes for breeding. They were known for savagery, and one of the artefacts that they had produced was a "beach ball" made of human skin of different races.
They also had the ability to preserve their consciousness during death and pass it to another living human. It was suggested Buddhist leaders who reincarnate, such as the Dalai Lama, were doing a similar thing.
An expedition was sent underground, during which a distress message was received from a Chinese submarine which had been sent some time in the future, and it was revealed that the leader of the subterranean humans had lived for thousands of years and was actually the historical basis for the Satan referred to in the Bible.