Legendary Resistance (3/Day).

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

Actions

Multiattack.

The dragon makes one Bite attack and two Claw attacks.

Bite.

Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 11 (2d10) poison damage.

Claw.

Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Tail.

Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 10 (1d8 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Change Shape.

The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses its action to end it.

Nightmare Breath (Recharge 5–6).

The dragon exhales a cloud of spores in a 90-foot cone. Each creature in that area must make a DC 19 Wisdom saving throw. On a failed save, the creature takes 49 (9d10) psychic damage, and it is frightened of the dragon for 1 minute. On a successful save, the creature takes half as much damage with no additional effects. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

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.

Commanding Spores. The dragon releases spores around a creature within 30 feet of it that it can see. The target must succeed on a DC 19 Wisdom saving throw or use its reaction to make a melee weapon attack against a random creature within reach. If no creatures are within reach, or the target can't take a reaction, it takes 11 (2d10) psychic damage.
Tail. The dragon makes one Tail attack.
Spore Salvo (Costs 2 Actions). The dragon releases poisonous spores around a creature within 30 feet of it that it can see. The target must succeed on a DC 19 Constitution saving throw or take 28 (8d6) poison damage and become poisoned for 1 minute. The poisoned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Lair Actions

On initiative count 20 (losing initiative ties), the dragon can take one of the following lair actions; the dragon can't take the same lair action two rounds in a row:

Deep Torpor. The dragon casts the slow spell, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 16). The spell ends early if the dragon uses this lair action again or if the dragon dies.
Mossy Sludge. The dragon conjures sludge-like moss that briefly covers surfaces in the lair. The ceiling, floor, and walls of the lair become difficult terrain until initiative count 20 on the next round.
Toxic Spores. The dragon fills a 20-foot cube it can see within 120 feet of itself with toxic spores. Each creature in that area must succeed on a DC 15 Constitution saving throw or take 14 (4d6) poison damage and be poisoned until the end of its next turn.

Regional Effects

The region surrounding a legendary deep dragon's lair is altered by the dragon's magic, creating one or more of the following effects:

Preservation of Knowledge. Books, letters, and any other physical forms of writing within 6 miles of the dragon's lair become magically charged and can't be damaged by nonmagical means.
Restless Sleep. When a creature finishes a long rest within 6 miles of the lair, the creature must first succeed on a DC 10 Constitution saving throw or be unable to reduce its level of exhaustion. Creatures immune to the poisoned condition are immune to this effect.
Verdant Growth. Vegetation and fungi within 6 miles of the dragon's lair grow faster and cover a greater area than they normally would. Foraging in this area yields twice the usual amount of food.

If the dragon dies, these effects fade over the course of 1d10 days.

Ancient Deep Dragon