Great Spirit Forest Aharthern (大精霊の森アハルトヘルン, Dai Seirei no Mori Aharutoherun) is the world where the spirits live. Its whereabouts change over time and can only be found by following rumors of the Great Spirit Forest.
2,000 years ago, Anos and his army invaded Aharthern with the intention of finding the Great Spirit Reno. With the defeat of its guardian, the Great Water Spirit Liniyon, the demon army was able to set the whole forest ablaze in order to draw out Reno.
Some time after Anos sacrificed himself to cast Four Worlds Wall <Beno Yeven>, an amnesiac Melheis Boran found himself in Aharthern, possibly trying to escape from whomever erased his memory.
Aharthern is a spirit formed by the rumor of a mysterious forest where the Great Spirit lives. This forest is alive and constantly moving, and its whereabouts and conditions to enter depend on rumors from time to time. Since Aharthern has no magic power, it cannot be seen with demon eyes.
- Web Novel Volume 4 Chapter 141