Despite their large size, deep dragons were slender and agile, allowing them to crawl through the tunnels of the Underdark. In the light, the dragons' scales were the color of amethyst, but in the dark, they appeared nearly black.  Deep dragons inhabited the Underdark and were rarely known on the surface.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:

• Stones erupt in a 20-foot radius centered on a point on the ground that the dragon can see within 120 feet of it. That area becomes difficult terrain, and each creature there must succeed on a DC 15 Dexterity saving throw or be knocked prone by the eruption. 
• A wall of mysterious fog seeps through the cracks of the cave within 120 feet of the dragon. The wall is up to 60 feet long, 10 feet high, and 5 feet thick, and it blocks line of sight. When the wall appears, each creature in its area must make a DC 15 Dexterity saving throw. A creature that fails the save takes 18 (4d8) psychic damage and is pushed 5 feet out of the wall’s space, appearing on whichever side of the wall it wants. A creature can move through the wall, albeit slowly and painfully. For every 1 foot a creature travels through the wall, it must spend 4 feet of movement. Furthermore, a creature in the wall’s space must make a DC 15 Intelligence saving throw once each round it’s in contact with the wall, taking 18 (4d8) psychic damage on a failed save, or half as much damage on a successful one. The wall dissipates when the dragon uses this lair action again or when the dragon dies.
• Magical fog billows around one creature the dragon can see within 120 feet of it. The creature must succeed on a DC 15 Wisdom saving throw or be charmed by the dragon until initiative count 20 on the next round.