Amphibious. The dragon can breathe air and water.

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

Undead Fortitude. If damage reduces the zombie 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 zombie drops to 1 hit point instead.

Multiattack. The dragon can use its Frightful Presence. It then makes five attacks: one with its bite and one for each on it's claws with its claws.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) acid damage.

Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) 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 18 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.

Acid Breath (Recharge 5–6). The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.

Create Undead Army (5 Day Recharge). The Dragon chooses up to three hundred corpses of Medium or Small humanoids within range. Each corpse becomes a ghoul under its control. (The GM has game statistics for these creatures.)

As a bonus action on each of it's turns, it can mentally command any creature it animated with this spell if the creature is within 120 feet of it (if it controls multiple creatures, the Dragon can command any or all of them at the same time, issuing the same command to each one). It can decide what action the creature will take and where it will move during its next turn, or it can issue a general command, such as to guard a particular chamber or corridor. If it issue's no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until its task is complete. The Zombies can not be told to go within a mile of where the dragons hollow actually is, out of the fear they felt for it in life.

The creature is under the Dragons control for 1 year, after which it stops obeying any command it has given it. To maintain control of the creature for another year, it must cast this spell on the creature before the current 1 year period ends. This use of the spell reasserts your control over up to three hundred creatures you have animated with this spell, rather than animating new ones.

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 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed. 

Cloud Kill. The Dragon exhales a 40-foot-radius sphere of poisonous, yellow-green fog centered on a point it chooses within range. The fog spreads around corners. It lasts for the duration or until strong wind disperses the fog, ending the spell. Its area is heavily obscured.

When a creature enters the spell's area for the first time on a turn or starts its turn there, that creature must make a Constitution saving throw. The creature takes 7d8 poison damage on a failed save, or half as much damage on a successful one. Creatures are affected even if they hold their breath or don't need to breathe.

The fog moves 10 feet away from the Dragon at the start of each of its turns, rolling along the surface of the ground. The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.

Life Leeching Bite. (Recharge 5-6) The Dragon makes a bite attack at a creature it chooses. On a hit the creatures hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Dragon regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.