Ancient Black Dragon

The ancient black dragon can breathe air and water.

3/day. If the ancient black dragon fails a saving throw, it can choose to succeed instead.

3 legendary actions; one at a time at the end of another creature’s turn, regaining spent uses at the start of its turn

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

Melee Weapon Attack: +15 to hit, reach 15 ft.. Hit: 19 (2d10 + 8) piercing plus 9 (2d8) acid damage.

Melee Weapon Attack: +15 to hit, reach 10 ft.. Hit: 15 (2d6 + 8) slashing damage.

Melee Weapon Attack: +15 to hit, reach 20 ft.. Hit: 17 (2d8 + 8) bludgeoning damage.

Recharge 5–6. The ancient black dragon exhales acid in a 90-foot line. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

Each creature of the ancient black dragon's choice that is within 120 ft. of the dragon and aware of it must succeed on a DC 19 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 ancient black dragon's Frightful Presence for the next 24 hours.

The ancient black dragon makes a Wisdom (Perception) check.

The ancient black dragon makes a tail attack.

Costs 2 legendary actions. The ancient black dragon beats its wings. Each creature within 15 ft. of the ancient black dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The ancient black dragon can then fly up to half its flying speed.