theory about forest of darkness
We know how almost all dangerous places in nameless1 were created.
Wind Island - created by sound of the winds with the help of wind elementals.
Castle of Light - created by Sheritt to protect children.
Gate to the Demon World - giant sinkhole which shares a name with a literal gate to the Demon World. It really does have ties to the Demon World. After all, there's a reason White Star used this location.
Valley of Despair - created by World Tree to protect itself.
Path of No Return - located in a rainforest. We don't know how the fog was created to make it so easy to get lost. Likely naturally created zone.
Land of Death - most likely created after the death of Queen of Death. Also Cage theorizes that it might be a blessing from the God of Death.
Gorge of Death - created by nature.
Forest of Darkness - has existed since ancient times and has ties to Black Mages.
We know almost nothing about the place Cale calls his backyard. There is a passage there connecting the two continents, through which monsters from the Eastern Continent are found in the forest. There are black trees, flora and fauna that are hard to find anywhere else. Monsters from two continents and mutants. In this place you could find dragon bones and a swamp full of dead mana created after its death. The forest is divided into regions, in the central regions there are stronger monsters that are not even afraid of stronger creatures like dragons.
In the second part we met the God of Chaos, we found out what kind of creatures he likes the most. Yes, mutants. Cale also had a theory that GoC was already working with the God of War in ancient times, because the chains that held Sky eating water captive were grey and the lake looked like an eye. In addition, we learned about one of his powers, which may be associated with the lack of fear in stronger beings in the forest. The passage between continents could have been GoC's doing. Because of this, some monsters may have mutated and only monsters can pass through this tunnel.
In addition to the black mages serving the Ancient White Star, among his people there may have been believers of the God of Chaos who were responsible for the experiments. The Forest of Darkness may have been created to do experiments, which is why it is such a mysterious and dangerous place.
I wouldn't be surprised if Cale found a ruined temple of the God of Chaos in his backyard. There is a good chance that they will need to go back to Nameless1 and search for information about ancient times.