Ancient Black Dragon

Gargantuan dragon, chaotic evil

Armor Class 22 (natural armor)

Hit Points 367 (21d20+147)

Speed 40ft., fly 80ft., swim 40ft.

Saving Throws Dex +9, Con +14, Wis +9, Cha +11

Skills Perception +16, Stealth +9

Damage Immunities acid

Senses blindsight 60ft., darkvision 120ft., passive Perception 26

Languages Common, Draconic

Challenge 21 (33,000 XP)

Traits

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 15ft., one target.
Hit: 19 (2d10+8) piercing damage plus 9 (2d8) acid damage.

Claw

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

Tail

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

Frightful Presence

Each creature of the dragon's choice that is within 120ft 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 90ft line that is 10ft 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 15ft 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.