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 movement.

Legendary Resistance (3/Day).

If the dragon fails a saving throw, it can choose to succeed instead.

Actions

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.

Cold Breath (Recharge 5–6).

The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 72 (16d8) cold damage on a failed save, or half as much damage on a successful one.

Legendary Actions

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.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

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:

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 dragon creates an opaque wall of ice on a solid surface it can see within 120 feet of it. The wall can be up to 30 feet long, 30 feet high, and 1 foot thick. When the wall appears, each creature within its area is pushed 5 feet out of the wall's space; appearing on whichever side of the wall it wants. Each 10-foot section of the wall has AC 5, 30 hit points, vulnerability to fire damage, and immunity to acid, cold, necrotic, poison, and psychic damage. The wall disappears when the dragon uses this lair action again or when the dragon dies.

Additional Lair Actions[–]

At your discretion, a legendary (adult or ancient) white dragon can use one or both of the following additional lair actions while in its lair:

Blinding Sleet. Driving sleet falls in a 40-foot-high, 20-foot-radius cylinder centered on a point the dragon can see within 120 feet of it. Each creature in that area must succeed on a DC 15 Constitution saving throw or be blinded until initiative count 20 on the next round.
Whirling Wind. A strong wind blows in a 30-foot-radius sphere centered on the dragon (see the Dungeon Master's Guide for rules on strong wind). The dragon's flying is not affected by this wind, which lasts until the next time the dragon uses a lair action or until the dragon dies.

Regional Effects

The region containing 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.

Additional Regional Effects[–]

Any of these effects might appear in the area around a white dragon's lair, in addition to or instead of the effects described in the Monster Manual:

Biting Chill. Extreme cold envelops the land within 6 miles of the dragon's lair (see the Dungeon Master's Guide for rules on extreme cold). If the climate in the area already features extreme cold, the cold is numbing, giving creatures in the area without immunity or resistance to cold damage disadvantage on Strength and Dexterity checks.
Mirror Ice. The icy surfaces in the dragon's lair reflect light like mirrors, giving creatures in the lair other than the dragon disadvantage on Dexterity (Stealth) checks made to hide. In addition, at each intersection or branching passage, any creature other than the dragon has a 50 percent chance of going in a different direction from the direction it intends.

Ancient White Dragon