It has nothing to do with the ruin chapel tunnel which use to save the bandit. There are 3 entrance for this cave, which only accessible once you turn into dragon knight, after talana death. One entrance is near a small goblin camp, which not further up when talana corpse, with a teleport shrine near by. Another entrance is at the bottom of lovis tower, which infested by skeletons. The 3rd entrance is by the cliff area at the northern part of broken valley map.
