Ancient Black Dragon
Gargantuan Dragon (chromatic), Chaotic EvilAC 22
HP 367 (21d20 + 147)
Speed 40 ft., fly 80 ft., swim 40 ft.
STR
27 (+8)
Save +8
DEX
14 (+2)
Save +9
CON
25 (+7)
Save +7
INT
16 (+3)
Save +3
WIS
15 (+2)
Save +9
CHA
22 (+6)
Save +6
Saving Throws DEX +9, WIS +9
Skills Perception +16, Stealth +9
Immunities Acid
Senses Blindsight 60 ft., Darkvision 120 ft., Passive Perception 26
Languages Common, Draconic
CR 21
Traits
Amphibious. The dragon can breathe air and water.
Legendary Resistance (4/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.
Actions
Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Melf's Acid Arrow (level 4 version).
Rend. Melee Attack Roll: +15, reach 15 ft. Hit: 17 (2d8 + 8) Slashing damage plus 9 (2d8) Acid damage.
Acid Breath (Recharge 5+). dex 22, each creature in a 90-foot-long, 10-foot-wide Line. Failure: 67 (15d8) Acid damage. Success: Half damage.
Legendary Actions
Cloud of Insects. dex 21, one creature the dragon can see within 120 feet. Failure: 33 (6d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dragon can't take this action again until the start of its next turn.
Frightful Presence. The dragon uses Spellcasting to cast Fear. The dragon can't take this action again until the start of its next turn.
Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.