The Forest of Terror is a dark and eerie forest located on a remote, unnamed island where Paradise Grass grows.
The Forest of Terror is located through a statue of an unknown creature's large mouth. Master Roshi and Fortuneteller Baba explain that the Forest of Terror is a special place between this world and the afterlife, and those who enter the forest will have their inner memories take physical form. The forest will create illusions of the trespassers' past opponents, who are apparently solid (as Goku tried to punch one but his punch was blocked, and he was knocked back). The more afraid the trespasser is, the larger the illusions will grow and the more they multiply, feeding onto the trespasser's inner emotions. The illusions will also grow larger if the trespasser uses ki in any way: such as transforming or flying (Goku uses Nimbus to counteract this).
The illusions cannot be defeated by any normal means as they will simply re-appear. The only way to defeat the illusions is to control your ki and suppress it, also suppressing your emotions, and the illusions will be able to be destroyed. While the illusions are active and whole, escape is impossible as the trees will instantly grow enough to where they block any exits.
While training and fetching the paradise grass for Master Roshi, Goku and Krillin enter the forest; they comment that the forest feels weird compared to the rest of the island and spooky. They are surrounded by illusions of Vegeta, Nappa, Frieza, Cell, Super Buu, the Ginyu Force, Dabura, Tambourine, King Piccolo, and even Bulma. Krillin, who was afraid the entire time, caused the illusions to grow and multiply, even to the point where Goku was overwhelmed. After becoming a Super Saiyan, even a Super Saiyan 3, Goku realizes the trick behind the illusions and suppresses his energy, causing them to disappear. Krillin, regaining his confidence and his composure, defeats more illusions himself. Baba then causes an illusion of Super Shenron to appear, and Goku (as a Super Saiyan Blue) and Krillin defeat it with a combined Kamehameha. With its defeat, the Paradise Grass grows, and Goku and Krillin take it back to Master Roshi.