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: +15 to hit, reach 15 ft., one target. Hit: 22 (2d12 + 9) piercing damage.

Claw

Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 18 (2d8 + 9) slashing damage, and the target loses 5 hit point from bleeding at the start of each of its turns for six rounds unless it receives magical healing. Bleeding damage is cumulative; the target loses 5 hp per round for each bleeding wound it's taken from a mithral dragon's claws.

Tail

Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) 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 its turn, 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)

A mithral dragon can spit a 60-foot-long, 5-foot-wide line of metallic shards. Targets in its path take 59 (17d6) magical slashing damage and lose another 10 hit points from bleeding at the start of their turns for 6 rounds; slashing and bleed damage is halved by a successful DC 21 Dexterity saving throw. Only magical healing stops the bleeding before 6 rounds. The shards dissolve into wisps of smoke 1 round after the breath weapon's use.