The Oni-Gara (2009)

Professor Asanuma leads a team of six folklorists, both men and women, searching for a site known as the Demon Shell Stone. Ignoring villagers' warnings, they climb a mountain in a village for investigation. However, upon discovering the Demon Shell Stone at the summit, one member suddenly goes mad, attacks, and devours Professor Asanuma. They attempt to descend immediately, but the cannibalism seems contagious; members rapidly lose their sanity.