Dark Forest
The Dark Forest is a spooky place. Darkwood Trees leave most sections of this biome as dark as underground. Above the trees, ponds, and, much less commonly, lava pools (which don't burn the leaves), can be generated, as well as the frightening Dark Tower and Goblin Knight Stronghold. The mobs found here are Skeletal Druid, Kobolds, Zombies, Skeletons, Mistwolves, and King Spiders. Dark Forests are the only place to find Dark Towers, Goblin Knight Strongholds, Mistwolves and King Spiders.
Progression
Using the progression system, the area beneath the trees in the Dark Forest will be filled with smoky ribbons of dark magic. You will be affected with blindness if you stay in the area. Defeating the Hydra will allow you to enter unimpeded. You will also need the Hydra Trophy to unlock the lower levels of the Goblin Knight Stronghold.
Defeating the Knight Phantoms will allow you to interact with and enter the Dark Tower at the center of the dark forest.