Ancient Gray Dragon

Spellcasting. Its spellcasting ability is wisdom (spell save DC 23, +13 to hit with spell attacks). 

At will; Mold Earth, Absorb Elements, Cause fear, Earth tremor, Meld into stone, Wall of stone, Invisibility

3/day; Fog Cloud (3rd level), Sleep (4th level), Erupting Earth,

1/day;

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

Action Melee Attack. Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) fire damage plus 21 (2d10+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 21 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.

Action Ranged Attack. Lava breath; 90ft cone, DC 24 dex save, 20d6 fire + 20d6 bludgeoning damage on failure, half as much on success. Recharge 5-6

Multiattack. The dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) fire damage plus 21 (2d10+10) bludgeoning damage.

The dragon has 3 legendary actions

Tail. (Costs 1 Action).Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Lava Breath (Costs 2 Actions). 90ft cone, DC 24 dex save, 20d6 fire + 20d6 bludgeoning damage on failure, half as much on success. Recharge 5-6.