You Will Return

After millennia in space, an offshoot of an advanced civilization returns to their home planet. Instead of the warm welcome they were hoping to receive, they discover that the planet has become uninhabitable. There are no signs of life anywhere on the surface, and not even ruins mark where their people once lived. Except... Except a vault is discovered! The walls inside are covered with the runes of an old language. Who built this structure? What is its purpose? Only by exploring the empty (?) h...