Dark Forest

A dark forest, or roofed forest, is a mild forested biome. It is known for its thick, dense leaf canopies that block light sufficiently to allow hostile mobs to spawn during daytime. Woodland mansions exist only in a dark forest. It has two variants, the normal dark forest, and dark forest hills.

Description
Dark forests feature 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 these biomes; the only place where dark oak trees naturally generate. Additionally, both types of huge mushrooms generate among the trees, though the red variants are more common. Like normal forests, lily of the valley, rose bushes, peonies and lilacs generate in this biome family along with the usual dandelions and poppies. Grass and leaves take on a dark but vibrant green color.

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 one of 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 dark forests, survival can still be far more difficult compared to 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 are dangerous to explore without adequate preparation, due to their sheer size and the difficult Illager and other 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 these biomes can help to clear out some trees due to their flammability.

Variants
There are two different dark forest variant biomes.

Dark Forest
Dark Forest is the most common dark forest variant. The terrain can range from fairly even to hilly, with occasional lakes. This biome has a dense canopy of leaves that reduces the light sufficiently for undead mobs to inhabit the forest during daytime. Small patches of plains biomes can generate clearings within the forest. Woodland mansions also generate here.

Dark Forest Hills
Dark Forest Hills can break the leaf canopy, increasing visibility and decreasing the chance of daytime hostile mob spawning, though the hills are steep compared to other hill biomes. Hills generated near rivers can lead to steep cliffs, risking fall damage if players don't land in the water. Small plains-biome clearings will not generate within the dark forest hills variant. Woodland mansions may still generate here.