The Miasma Forest's title originates from the legend of Zanaffar. When the Swordsman of Light defeated Zanaffar, it was said that the beast bled so profusely that the nearby forest was entrenched in it, making it into a lake of blood. This was temporary, however, and the blood eventually receded. However, the forest was left with a strange presence surrounding it, likely due to Zanaffar's miasma that was also connected to the growth of the holy tree Flagoon. The other theory for this strange presence's origin are the numerable crimes that have been committed in the forest over the years have originated it.
The presence hanging over the forest causes most every sound to be silenced or muffled; and there are either no animals living in the forest, or they are just never heard or seen. This is never made exactly clear in the novels. However, in the anime, normal animal sounds can be heard. The presence also keeps anyone from detecting the auras of other people, so detecting an enemy before a strike is nearly impossible. Thus, killing someone in the forest is made that much easier, that is, if one can find the target.