The Forest World is one of the 12 original worlds accessible directly from The Nexus. It is located at the 11:30 position, and it is one of the most basic worlds in the game.
Features
A Forest area with a part of the Aztec Rave Monkey as its Parallax Background. There are several purple or pink slug-like blobs placed randomly in the forest that do nothing to you when touched. However, if they are interacted with, they'll make a sound and a wobbly transparent image of them will begin to hover above Madotsuki's head.
The Frog Effect is available in this world. Look for a frog hopping around between trees in the upper left area.
Connections
Face Carpet Plaza - Look for a set of green pillars towards the upper right. There is a block portal to Hell there, as well as a Lunatic Toriningen.