Undead Fortitude. If damage reduces the dragon to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the dragon drops to 1 hit point instead.
Incorporeal Movement. The dragon can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
Incorporeal Body. Enemies attacking the dragon have disadvantage on their attack rolls. True Sight negates the penalty.
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: +12 to hit, reach 15 ft., one target. Hit: 17 (2d10 + 5) piercing damage.
Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 5) slashing damage.
Tail. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 15 (2d8 + 5) bludgeoning damage.
Life Drain. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 23 (4d8 + 5) necrotic damage. The target must succeed on a DC 17 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
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 24 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.
Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.
Cold Breath. The dragon exhales cold in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 71 (13d10) cold damage on a failed save, or half as much damage on a successful one.
Ghastly Breath. The dragon exhales a ghastly fog in a 90-foot cone that lingers for 5 (1d4 + 3) turns. Each creature in that area must succeed on a DC 24 Constitution saving throw or gain 1 level of exhaustion. Any creature ending its turn within the fog must succeed on a DC 20 Constitution saving throw or gain 1 level of exhaustion.
Furious Breath. If the dragon drops to half hit points or below it immediately exhales its Ghastly Breath in the direction of its attacker.
The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option 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 25 Dexterity saving throw or take 12 (2d6 + 5) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.