Undead Fortitude. If damage reduces the undead dragon to 0 hit points, it must make a Constitution saving throw with a DC equal to the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Legendary Resistance (1/Day). If the undead dragon fails a saving throw, it can choose to succeed instead.

Magic Resistance. The undead dragon has advantage on saving throws against spells and other magical effects.

Multiattack. The undead dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage plus 7 (2d6) poison damage.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) slashing damage.

Poison Breath (Recharge 5–6). The undead dragon exhales poisonous gas in a 30-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 56 (16d6) poison damage on a failed save, or half as much damage on a successful one.

The undead 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 undead dragon regains spent legendary actions at the start of its turn.

Claws. The undead dragon makes one attack with its claws.

Frightful Presence. Each creature of the undead dragon's choice that is within 120 feet of the undead dragon and aware of it must succeed on a DC 13 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 undead dragon's Frightful Presence for the next 24 hours.

Undead Regeneration (Costs 2 Actions). The undead dragon regains 29 (3d10 + 13) hit points. If the undead dragon takes radiant or fire damage, this action only regenerates half as much until the start of the its next turn.