Ancient Blue Dragon

From Monster Manual, page 90.

**Hit Points:** 481 (26d20 + 208)
 **STR**
29 (+9)
10 (0)
27 (+8)
18 (+4)
17 (+3)
 **Saving Throws:** Dex +7, Con +15, Wis +10, Cha +12
**Skills:** Perception +17, Stealth +7
**Damage Immunities:** lightning
**Senses:** blindsight 60 ft., darkvision 120 ft., passive Perception 27
**Languages:** Common, Draconic
 ***Legendary Resistance (3/Day).*** If the dragon fails a saving throw, it can choose to succeed instead.
 ***Multiattack.*** The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.
***Bite.** Melee Weapon Attack:* +16 to hit, reach 15 ft., one target. *Hit:* 20 (2d10 + 9) piercing damage plus 11 (2d10) lightning damage.
***Claw.** Melee Weapon Attack:* +16 to hit, reach 10 ft., one target. *Hit:* 16 (2d6 + 9) slashing damage.
***Tail.** Melee Weapon Attack:* +16 to hit, reach 20 ft., one target. *Hit:* 18 (2d8 + 9) bludgeoning damage.
***Frightful Presence.*** Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 20 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.
 The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.
**Detect.** The dragon makes a Wisdom (Perception) check.
**Tail Attack.** The dragon makes a tail attack.
 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:
* Part of the ceiling collapses above one creature that the dragon can see within 120 feet of it. The creature must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) bludgeoning damage and be knocked prone and buried. The buried target is restrained and unable to breathe or stand up. A creature can take an action to make a DC 10 Strength check, ending the buried state on a success.
* A cloud of sand 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 region surrounding a legendary blue dragon's lair is warped by the dragon's magic, which creates one or more of the following effects:
* Thunderstorms rage within 6 miles of the lair.
* Dust devils scour the land within 6 miles of the lair. A dust devil has the statistics of an Air Elemental, but it can't fly, has a speed of 50 feet, and has an Intelligence and Charisma of 1 (-5).
* Hidden sinkholes form in and around the dragon's lair. A sinkhole 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 sinkhole must succeed on a DC 15 Dexterity saving throw or fall 1d6 × 10 feet into the sinkhole.

If the dragon dies, the dust devils disappear immediately, and the thunderstorms abate within 1d10 days. Any sinkholes remain where they are.