Deep dragons make their lairs in hidden caves in the Underdark, using their large but slender and agile bodies to crawl through the narrow tunnels of the Underdark.

Ominous psionic power emanates from a legendary deep dragon's lair. The sheer void of the Underdark offering plenty of traps and defenses should a deep dragon feel threatened.

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:

• Sheer insanity affects one creature within 12o feet of the dragon. The creature must succeed on a DC 17 Wisdom saving throw or take 10 (3d6) psychic damage and begin attacking random creatures in range. The affected creature continues attacking random creatures in range until 10 minutes have passed, or a friendly creature deals non-lethal damage to the affected creature. A creature can take an action to make a DC 14 Wisdom check, ending the insanity state on a success.
• A cloud of dark swirls about in a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The cloud spreads around corners. Each creature in the cloud must succeed on a DC 15 Constitution saving throw or be blinded for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
• The pure psionic power of the dragon causes all creatures within the lair to make a DC 16 Wisdom saving throw or begin attacking themselves, taking 1d6+STR modifier damage at the end of each of their turns. A creature can take an action to make a DC 14 Wisdom check, ending the self-damaging state. 

Regional Effects

The region containing a legendary blue dragon’s lair is warped by the dragon’s magic, which creates one or more of the following effects:

• Creatures often forget important items or ideas within 6 miles of the lair.
•  Giant Spiders scour the land within 6 miles of the lair.
• Hidden spike traps form in and around the dragon’s lair. A spike trap can be spotted from a safe distance with a successful DC 20 Wisdom (Perception) check. Otherwise, the first creature to step on the thin crust covering the spike trap must succeed on a DC 15 Dexterity saving throw or take 5d6 piercing damage.

If the dragon dies, the giant spiders disappear immediately, and the creatures are no longer affected by the dragons psionic power within 1d10 days. Any spike traps remain where they are.