Ancient Black Shadow Dragon

Amphibious: The dragon can breathe both air and water.

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

Living Shadow: While in dim light or darkness, the dragon has resistance to damage that isn't force, psychic, or radiant.

Magic Weapons: The dragon's weapon attacks are treated as magical weapons.

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

Shadow Stealth: While in dim light or darkness, the dragon can take the Hide action as a bonus action.

Strafing: When the dragon is flying, it can add its movement to the length of its breath weapon

Sunlight Sensitivity: While in sunlight, the dragon has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Multiattack: The dragon can use its Frightful Presence. It can then make three attacks: two with its claws and one with its bite.

Bite: Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 8) piercing damage plus 18 (4d8) necrotic damage.

Claw: Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 22 (4d6 + 8) slashing damage.

Tail: Melee Weapon Attack: +15 to hit, reach 30 ft., one target. Hit: 26 (4d8 + 8) bludgeoning damage. If the target is a creature, it must succeed on a DC 23 Constitution saving throw or be knocked prone or pushed 20 ft., the dragon's choice, and stunned until the end of the dragon's next turn.

Frightful Presence: Each creature of the dragon's choice that is within 120 ft. of the dragon and aware of it must succeed on a DC 19 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 Weapon (Recharge 5-6): The dragon uses one of the following breath attacks:

Acid Breath: The dragon exhales acid in a 120-ft. line that is 10 ft. wide. Each creature in that area must make a DC 22 Dexterity saving throw, taking 99 (22d8) necrotic damage on a failed save, or half as much damage on a successful one. 

Shadow Breath: The dragon exhales a dark blast in a 120-ft. line that is 10 ft. wide. Each creature in that area must make a DC 22 Dexterity saving throw, taking 99 (22d8) necrotic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies, and an undead shadow (CR 1/2) rises from its corpse and acts immediately after the dragon in the initiative count. The shadow is under the dragon's control.

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 can make a Wisdom (Perception) check.

Tail Attack: The dragon makes a Tail attack.

Wing Attack (2 Actions): The dragon beats its wings. Each creature within 15 ft. of the dragon must succeed on a DC 23 Dexterity saving throw or take 22 (4d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.