Ancient Black Dragon

Chromatic Dragon

Gargantuan dragon, chaotic evil

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

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)

Abilities

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.