Ancient Black Dragon

Gargantuan dragon, chaotic evil

Armor Class 22 (natural armor)
Hit Points 367 (21d20 + 147)
Speed 40 ft., fly 80 ft., swim 40 ft.

STR: 27 (+8)
DEX: 14 (+2)
CON: 25 (+7)
INT: 16 (+3)
WIS: 15 (+2)
CHA: 19 (+4)

Saving Throws: Dex +9, Con +14, Wis +9, Cha +11
Skills: Perception +16, Stealth +9
Damage Immunities: acid
Senses: blindsight 60 ft., darkvision 120 ft., passive Perception 26

Languages: Common, Draconic

Challenge 21 (33,000 XP)

Amphibious: The dragon can breathe air and water.
Legendary Resistance (3/Day): If the dragon fails a saving throw, it can choose to succeed instead.

Actions
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: 19 (2d10 + 8) piercing damage plus 9 (2d8) acid damage.

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

Tail: Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) 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 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 dragon’s Frightful Presence for the next 24 hours.

Acid Breath (Recharge 5-6): The dragon exhales acid in a 90-foot line that is 10 feet wide. 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.

Legendary Actions
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 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked Prone. The dragon can then fly up to half its flying speed.