"This is where Eyes of the Forest sank into sightless night. These woods were always dark, but the Decay made their darkness deeper... and more dangerous."—Voice of the Forest, about Mouldwood Depths
The entrance to Mouldwood Depths is found in Kwolok's Hollow, below the place where Kwolok lives. Upon arriving, some Moki will speak to Ori, claiming that Kwolok sent them. After speaking of the Eyes of the Forest being somewhere in the den, they will then pull a lever on the statue of the spider, opening the door to Mouldwood Depths, while warning Ori to not get trapped in the darkness.
In Mouldwood Depths, there are certain places where the screen is darker than usual, to the point of having no light except for the glow of Ori and any nearby enemies or projectiles. In these cases, the player has to either follow a firefly or dash to the nearest area with light. If the player stays too long in the darkness, the glowing aurora around Ori will shrink until it totally disappears, leaving the player in complete darkness and finally, death. At some point player gets the Flash ability, which allows to create an aura around Ori and wander safely through the darkness.
Ori must venture deep into Mouldwood Depths in order to find the Eyes of the Forest that has fallen there, and battle the spider Mora to release her from the darkness that has controlled her mind. After Mora is defeated, she will thank Ori and let loose the wisp, saying that even those beneath the surface need the light to provide balance.