The lands that made up the trial by fire were split into four major areas, a desert, a swamp, a plain, and a jungle. They didn't interlock, but rather, were separated by enormous mountain ranges and other terrain features.
Although the lands were not endless, they were certainly vast.
Furthermore, hidden dangers lurked everywhere. That included both natural occurrences like the sandstorm, and bizarre entities of every sort. All of them could be extremely deadly.
Before entering the trial by fire, the cultivators from the four major rivers had all assumed it would be dangerous. Even still, now that they were inside, they were completely shocked by what they were facing.
In fact, the word 'danger' couldn't be used to describe the place. It was more like... a land of death!
