Trapping cave tunnels

Joren, Riven, and Taya had been friends since childhood. Over time, Joren and Taya even became a couple, while Riven still dwells on the terrible experiences of recent years. They had already been through a lot before. However, there was once a fourth person in their group whom none of them can remember today. A person who still knows everything about each of them and will not rest until every one of them stops breathing. A person they will soon encounter and wish they had died much earlier instead of surviving. He returns to finish what he started...