Forgotten Forest is a location in Phoenotopia. It is a vast area that covers half of the northern part of Castland. When it's first unlocked, the player will only have access to a very small portion of the forest. They can gain access to the rest of the level after delivering 30 Moonstones to Fran. This is a bonus area and has no impact on the main story, although completing it will unlock the Hidden Village where you can buy some very powerful items like the Blood Ring and Bottles of Glowy Wisps.
The Forgotten Forest is located in the northern part of the country and is initially inaccesable. The Forest is made of red maple trees and is primarily inhabited by Arcs, Harpies and Plantdogs. There are also signs of an ancient civilization as well and some of the structures are identical to the ones in Panselo, which indicates that humans used to inhabit this area centuries ago. Hidden Village is located at the end of the Forest.