Looking for this BL

I dont remember much, the mc went on a mission alone for someone in the middle of a forest/mountains and theres a traditional building in the middle. Theres supposed to be some sort of treasure hidden there. Some mysterious stuff is going on and its said no one came back alive after entering that area/building. The ML seems to be some sort of ancient snake? There was an old tale about betrayal with a gaint snake and a human. There was also a coffin with a body inside.