The Dark Forest is a biome added by Twilight Forest. This biome is completely covered in a 6-12 block thick roof of Darkwood Leaves. This biome is very dangerous, as there is next to no light inside. The only light comes from occasional Fireflies and Mushglooms. Additionally, Mist Wolves, Witches, Kobolds, King Spiders, and various vanilla monsters spawn inside here. The Goblin Knight Stronghold is located somewhere in the outskirts of the biome and the Dark Tower is located in the red-orange center, which houses the biome's boss, the Ur-ghast.
If the player gets to the Dark Forest without first killing the Lich, they will see black wavy lines on the edge of the forest. If the player gets to the Dark Forest without first killing the Hydra, they will see black wavy lines on the edge of the red-orange center forest. Entering the forest while the wavy lines are active will inflict Blindness on the player indefinitely until they leave. Killing the Hydra will lift the curse and make it possible to explore and interact with.