The Forests of Darkness

Hidden away in many deep forests of the world there exists regions of ancient growth and ancient magics. Rarely if ever encountered in forests of Europe is overgrown tears in reality created by any number of reasons. Most are caused by one of two reasons. 
THe first is the obvious release of powerful magic. This can be caused the use of magical spells, or the death of a creature filled with magical energy. Sometimes this is caused by the release of magical energy from strange substances falling to earth from beyond the planet itself. Meteorites & asteriods happen all the time and all it takes is one carrying specific magical substances that reacts violently when it hits the surface.
The second is that of a tear caused by creatures or items crossing from one dimension to another. Given enough time and use, this leaves an imprint that eventually widens & forms a permanent effect on the region.
Whatever the reason, these can cause the plant & animal life in the region to take on different attributes. Most adapt over time and take on strange otherworldly aspects. Many survive with little difference, and minor changes, though a few will take on far more pronounced changes. These can take on aspects of other organisms in the same area, others grow larger, or change in size getting larger or smaller over time. Sometimes the effects of other dimensions cause psychological changes too, increasing or deceasing intelligence, becoming more or less aggressive, or any other multitude of traits.
Those that seek out such places are usually after specific resources, including the rare instances where the very soil & bedrock is warped and changed. Others who stumble in find the areas affecdted strange and at times alien. In both cases, staying too long warps them mental and/or physically too. Both will find such places deadly in the long run as most harbour deadly microorganisms, plants & animals the creatures have no defences against.
To search for the forests of darkness one must listen carefully to local stories of strange occurances and life that seems out of place. Most have a story concerning some monstrosity that has wandered out of the forest and caused major problems all around the forest of darkness. If investigated, sometimes a few interesting resources can be found in the form of living organisms, or strange materials not native to this world.
It is estimated there is at least two such regions at least in Europe, one in the Black Forest n the Frankish Empire, and one in the Bohemian Massif to the east on their border areas. Hardly anyone has returned from trying to find them, and little in the way of peoples living near them to bother.