Talk:Caves/@comment-24277994-20131224182505/@comment-3284423-20131224194559

Those are nightmare fissures. Though they mostly occur in ruins they do occur less frequently in caves. The cave version looks different and produces fewer nightmare creatures if I remember correctly. These fissures are the only way to tell if the ruins are in the nightmare stage without actually being in the ruins, since the medallion won't work anywhere but the ruins.