Ancient White Dragon

From Monster Manual, page 100.

**Hit Points:** 333 (18d20 + 144)
 **STR**
26 (+8)
10 (0)
26 (+8)
10 (0)
13 (+1)
 **Saving Throws:** Dex +6, Con +14, Wis +7, Cha +8
**Skills:** Perception +13, Stealth +6
**Damage Immunities:** cold
**Senses:** blindsight 60 ft., darkvision 120 ft., passive Perception 23
**Languages:** Common, Draconic
 ***Ice Walk.*** The dragon can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra moment.
***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:* +14 to hit, reach 15 ft., one target. *Hit:* 19 (2d10 + 8) piercing damage plus 9 (2d8) cold damage.
***Claw.** Melee Weapon Attack:* +14 to hit, reach 10 ft., one target. *Hit:* 15 (2d6 + 8) slashing damage.
***Tail.** Melee Weapon Attack:* +14 to hit, reach 20 ft., one target. *Hit:* 17 (2d8 + 8) 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 16 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:
* Freezing fog fills a 20-foot-radius sphere centered on a point the dragon can see within 120 feet of it. The fog spreads around corners, and its area is heavily obscured. Each creature in the fog when it appears must make a DC 10 Constitution saving throw, taking 10 (3d6) cold damage on a failed save, or half as much damage on a successful one. A creature that ends its turn in the fog takes 10 (3d6) cold damage. A wind of at least 20 miles per hour disperses the fog. The fog otherwise lasts until the dragon uses this lair action again or until the dragon dies.
* Jagged ice shards fall from the ceiling, striking up to three creatures underneath that the dragon can see within 120 feet of it. The dragon makes one ranged attack roll (+7 to hit) against each target. On a hit, the target takes 10 (3d6) piercing damage.
 The region surrounding a legendary white dragon's lair is warped by the dragon's magic, which creates one or more of the following effects:
* Chilly fog lightly obscures the land within 6 miles of the dragon's lair.
* Freezing precipitation falls within 6 miles of the dragon's lair, sometimes forming blizzard conditions when the dragon is at rest.
* Icy walls block off areas in the dragon's lair. Each wall is 6 inches thick, and a 10-foot section has AC 5, 15 hit points, vulnerability to fire damage, and immunity to acid, cold, necrotic, poison, and psychic damage.
* If the dragon wishes to move through a wall, it can do so without slowing down. The portion of the wall the dragon moves through is destroyed, however.

If the dragon dies, the fog and precipitation fade within 1 day. The ice walls melt over the course of 1d10 days.