Ancient Black Dragon

Gargantuan dragon · CR 21

Armor Class 22

Hit Points 367 (21d20+147)

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

STR 27 +8

DEX 14 +2

CON 25 +7

INT 16 +3

WIS 15 +2

CHA 19 +4

Saving Throws CHA+11, CON+14, DEX+9, WIS+9

Skills perception +16, stealth +9

Damage Immunities acid

Senses blindsight60 ft., darkvision120 ft. , passive Perception 26

Languages Common, Draconic

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/Ranged Weapon Attack: +15 to hit.: Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) acid damage.
Claw. Melee/Ranged Weapon Attack: +15 to hit.: Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.
Tail. Melee/Ranged Weapon Attack: +15 to hit.: 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 Ancient Black 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 creature 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) (Costs 2 Actions).: The dragon beats its wings. Each creature within 15 ft. 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.