The Dark Forest biome (known as the Roofed Forest biome before 1.13) is a lush forested biome. It is known for its thick, dense leaf canopy. It has two variations in total.
The dark forest features a much higher density in trees compared to other forest biomes, which oftentimes block direct views of the sky entirely. Oak, dark oak, and (rarely) birch trees generate in this biome; this is the only biome where dark oak trees naturally generate. Additionally, both types of huge mushrooms generate among the trees in this biome, though the red variants are more common. Like normal forests, rose bushes, peonies and lilacs generate in this biome along with the usual dandelions and poppies. Grass and leaves take on a dark but vibrant green color.
The terrain is somewhat hilly, with lakes generating every so often. Lava lakes that generate in dark forests can cause locally destructive forest fires, due to the extreme density of flammable wood and leaves; the same goes for lightning strikes that hit the trees during thunderstorms, though the fires caused by the strikes are usually quickly put out by the rain.
Unique to dark forests are woodland mansions, enormous dungeon-like structures that very rarely generate within the trees. Woodland mansions are home to the dangerous Illager mobs, and are the only places where said mobs spawn. Mansions contain various rooms and valuable loot for the player to plunder.
While players will likely never run out of wood in this biome, survival can still be far more difficult compared to the other forest biomes. The dense leaf canopy can often diffuse sky light to the point that hostile mobs can spawn during the day in certain spots. Additionally, hostile mobs can spawn atop huge mushrooms and surprise the player when they suddenly fall down near them. Woodland mansions can be very dangerous to explore if not adequately prepared, due to their sheer size and the difficult Illager mobs accompanied by typical hostile mobs inside. The high density of trees can easily lead to players getting lost if they don't mark their trails using torches or other distinct markers. Using flint and steel in this biome can help to clear out some trees due to their flammability.
Dark forests only have one variant, for a total of two different dark forest biomes.
Dark Forest Hills
Dark Forest Hills (known as Roofed Forest M before 1.13) can break the leaf canopy, increasing visibility and decreasing the chance of daytime hostile mob spawning, though the hills are quite steep compared to other hill biomes. Hills generated near rivers can lead to steep cliffs, risking potentially fatal fall damage if players don't land in the water. Woodland mansions may still generate here.
A dark forest with a lake.
A dark forest that failed to generate correctly due to water.
Another image of dark forest hills.
A dark forest bordering a swamp.
A woodland mansion in a dark forest.