Great Forests are areas of Polemos where the natural order is weakened to the point that celestial beings run amok, drawn to the new space like moths to a flame. These areas become filled with Chaos, with the natural world reacting by growing huge, dark, twisted forests. The exact details differ from forest to forest, but most feature trees hundreds of feet tall, pitch darkness, malicious spirits (called Dark Ones), and monstrous corruptions of natural creatures.
Great Forests are presumed to have formed as a part of The Worldbreak, severly hampering humanity's recovery after the cataclysm. Though they would slowly dwindle over time, even after millennia the Great Forests would act as dangerous, impassable barriers blocking the expansion of mankind.