Ancient grey dragon

Voice mimic. The grey dragon can mimic any voice they have heard before perfectly.

Reflective Carapace. Any time the grey dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the grey dragon is unaffected. On a 6, the grey dragon is unaffected, and the effect is reflected back at the caster as though it originated from the grey dragon, turning the caster into the target. 

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

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

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., 1 target. Hit: 23 (2d12 + 10) piercing damage and on a critical hit the target takes an extra 14 (4d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the grey 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.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., 1 target. Hit: 19 (2d8 + 10) slashing damage. 

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., 1 target. Hit: 21 (2d10 + 10) slashing damage. 

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 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.

Paralyzing Breath (Recharge 5-6). The grey dragon exhales a paralyzing mist in a 90ft cone. Each creature in that area must make a DC 25 Constitution saving throw or be paralyzed 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.

The ancient grey 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 ancient grey 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 ft. 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.