In the Nightmare Hunts, the cloudy gray barriers that block the path forward are dropped when you kill all enemies in the area. However, if you kill all enemies, then walk to another area that you load in, the barrier reappears when you reenter the original area. However, no enemies spawn to allow you to drop the barrier again. This specifically happened to me during [spoiler]the Phogoth hunt,[/spoiler] after I traveled from the Circle of Bones to the Chamber of Night and back again. I'm not sure if it's specifically with this area, all Nightmare Hunts, or if it was just a one time thing, but will update if I run another Nightmare Hunt.