Making their lairs in the depths of the Underdark, deep dragons are nightmarish cousins of chromatic dragons. The warped magical energy of their subterranean realm gives them the ability to exhale magical spores that instill fear and scar the mind.
Deep dragons’ black-and-gray hide is smooth like a salamander’s, and their eyes are pale. As they age, their spore breath causes fungi to bloom across their skin, especially around the head and neck. Their wings are attached to their front legs and can fold in close to the body, allowing deep dragons to easily maneuver through relatively narrow tunnels.
Deep dragons often hoard secrets, delighting in knowledge of far-off lands. Many seek out new insights and tricks that they can use against other denizens of the Underdark, preferring social manipulation and crafty dealmaking to exerting themselves in combat. Deep dragons look down on any creature that isn’t useful to them, though they are willing to bargain for knowledge they lack.
Deep dragons make their lairs in well-hidden caves or sunless beaches in the Underdark, and these sites are often inaccessible without the ability to fly or dive underwater. They fill their lairs with secret passages and hiding places that allow them to escape or ambush visitors if the need arises. A well-cultivated lair abounds with Underdark fungi and plants, with the floor, walls, and ceiling covered in carpets of mold and moss, or featuring larger mushrooms and plants in neatly organized displays.
The challenge rating of a legendary deep dragon increases by 1 when it’s encountered in its lair.
Deep Torpor. The dragon casts the slow spell, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 16). The spell ends early if the dragon uses this lair action again or if the dragon dies.
Mossy Sludge. The dragon conjures sludge-like moss that briefly covers surfaces in the lair. The ceiling, floor, and walls of the lair become difficult terrain until initiative count 20 on the next round.
Toxic Spores. The dragon fills a 20-foot cube it can see within 120 feet of itself with toxic spores. Each creature in that area must succeed on a DC 15 Constitution saving throw or take 14 (4d6) poison damage and be poisoned until the end of its next turn.
The region surrounding a legendary deep dragon’s lair is altered by the dragon’s magic, creating one or more of the following effects:
Preservation of Knowledge. Books, letters, and any other physical forms of writing within 6 miles of the dragon’s lair become magically charged and can’t be damaged by nonmagical means.
Restless Sleep. When a creature finishes a long rest within 6 miles of the lair, the creature must first succeed on a DC 10 Constitution saving throw or be unable to reduce its level of exhaustion. Creatures immune to the poisoned condition are immune to this effect.
Verdant Growth. Vegetation and fungi within 6 miles of the dragon’s lair grow faster and cover a greater area than they normally would. Foraging in this area yields twice the usual amount of food.
If the dragon dies, these effects fade over the course of 1d10 days.