The Shadowlands was the ground level of Kashyyyk, where katarns and other creatures lived, including at least one terantatek that lived during the times of the Jedi Civil War.
The Shadowlands were a dangerous place for unprepared travellers, as it hosted a variety of dangerous creatures, including Mandalorians, at least before Revan revisited the forest floor. There were many dark and mysterious secrets in this area of Kashyyyk, including an ancient Rakatan Star Map. A Rakatan computer near the Star Map guarded the Map itself and stored data about Kashyyyk. It was there that it was learned that the reason the trees of Kashyyyk were so tall was most likely because of Rakatan experiments.