Ancient Red Shadow Dragon

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

Limited Amorphousness. The dragon can move through a space as narrow as 2 feet wide without squeezing.

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

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

Sunlight Hypersensitivity. While in sunlight, the dragon takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it 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 then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 22 (2d10 + 11) piercing damage plus 14 (4d6) necrotic damage.

Rending Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 18 (2d6 + 11) slashing damage, and the target's Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

If a non-evil humanoid dies from this attack, a new shadow rises from the corpse 1d4 turns later under the dragon's control and acts immediately after the dragon in the initiative count.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 20 (2d8 + 11) 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 22 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.

Shadow Breath (Recharge 5–6). The dragon exhales shadowy fire in a 90-foot cone. Each creature in that area must make a DC 25 Constitution saving throw, taking 91 (26d6) necrotic damage on a failed save, or half as much damage on a successful one. 

If a non-evil humanoid dies from this attack, a new shadow rises from the corpse 1d4 turns later under the dragon's control and acts immediately after the dragon in the initiative count.

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.

Bite Attack. The dragon makes a bite attack.

Shadow Breath (Costs 2 Actions). The dragon makes a Shadow Breath, if it can.

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 18 (2d6 + 11) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.