One way of getting here, The Concave, is through conflagration of your Soul. Not a nice way to arrive, but at least you'll have the benefit of a guide, unlike most others.
As they travel they notice a horrible rotten smell. As they travel it gets stronger and stronger forcing them to cover their noses and mouth less they choke on the horrible smell. Stream full of dead fish. Spawning trout die in mass quantities after they spawn littering areas with dead fish dying and deteriorating in the sun.