Ancient adamantine dragon

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

Multi attack. The dragon can use its frightful presence and 1 bite attack plus 2 claw attacks.

Bite. Melee Weapon Attack: +18 to hit, reach 20 ft., 1 target. Hit: 25 (3d10 + 10) piercing damage plus 15 (5d6) fire damage

Claw. Melee Weapon Attack: +18 to hit, reach 15 ft., 1 target. Hit: 19 (3d6 + 10) [Slashing] damage. 

Tail. Melee Weapon Attack: +18 to hit, reach  25ft., 1 target. Hit: 22 (3d8 +10 ) [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 27 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.

Magical Fire Breath (Recharge 5-6). The dragon exhales magical fire in a 180-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 102 (34d6) fire damage on a failed save, or half as much damage on a successful one. This damage bypasses fire resistances but creatures with magic resistance makes the save with advantage.

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 15 ft. of the dragon must succeed on a DC 28 Dexterity saving throw or take 19 (3d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.