Undead Fortitude

If reduced to 0 HP, 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 Zombie Dragon drops to 1 HP instead.

Legendary Resistance (3/Day)

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

Multiattack

The dragon can use its Frightening Presence. It then makes three attacks: one with its bite and two with its claws.

Bite

Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2D10 + 5) piercing damage plus 4 (1D8) Necrotic damage.

Claw

Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 8 (2D6 + 5) slashing damage.

Tail

Melee Weapon Damage: +10 to hit, reach 15 ft., one target. Hit: 10 (2D8 + 5) 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 21 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.

Poison Breath (Recharge 5-6)

60 foot cone of poison, DC 17 Constitution saving throw or take 54 (12D8) on a failed save or half as much on a successful save.

Necrotic Breath (Recharge 5-6)

60 foot cone of Necrotic Miasma, DC 17 Wisdom saving throw or take 54 (12D8) on a failed save or half as much on a successful save.

The Zombie 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 Zombie 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 17 (2D6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.