The darkness in Aufstrag is not as normal darkness one might find under the ground. It is almost palatable. One can feel it as an oppressive weight that bares down upon you. It comes in the form of a mindless fear that creeps around the shadows of one's thought, a tiny voice in the mind, causing alarm. The longer one is in Aufstrag, the heavier the weight of it becomes, and the more raw the fear, the voice of which becomes louder.
Traveling in Aufstrag is difficult. For each day that someone lingers there they must make a successful primary attribute check (CL 10) or suffer from the weight of it. If they succeed there are no ill affects and they gain a bonus 1 to their next day's check, up to a maximum of +5. Paladins gain a +5 on their attribute check and give anyone in the party a +1. If one fails their check, then the fear of Aufstrag has settled into their mind and nothing removes it short of a remove fear or restoration spell, potion, ring, etc. It lingers even after one leaves Aufstrag. This is called the “clinging darkness.”