Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Nightmarishly Tough (Once a day). The zombie dragon is a personification of the word resilience. If damage reduces the zombie dragon to 0 hit points, Instead of dying it instead restores 8d12+20 (72) worth of hit points.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its Rotting Maw and two with its claws.

Rotting Maw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage plus 20 (2d10 + 9) Necrotic. After damage the target must make a DC 18 Constitution saving throw or given a random disease (DMs digression). 

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

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 11 (2d8 + 2) 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.

Diseased Breath (Recharge 5–6). The dragon exhales a liquid concoction of repugnant bodily fluids and rancid air in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Constitution saving throw, taking 77 (14d10) Necrotic damage on a failed save, or half as much damage on a successful one. And also receives a random disease (DMs digression). 

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 10 feet of the dragon must succeed on a DC 20 Dexterity saving throw or take 7 (1d6 + 4) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.