Ancient Black Dragon
Gargantuan Dragon, chaotic evil
33,000 XP
Special 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
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 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.
Quick Reference
Running This Encounter
Position the Ancient Black Dragon in its swamp lair where it can utilize water terrain for advantage. Use its legendary actions to maximize turn economy and threaten multiple party members. Employ minions like Lizardfolk or Giant Crocodiles to control battlefield positioning. Retreat to deeper water or caves if the dragon drops below 100 HP to force the party to pursue into disadvantageous terrain.
Combat Tactics
The dragon opens with its lightning breath to damage multiple targets before closing to melee range. It uses its water-breathing and swamp mobility to control engagements, focusing fire on spellcasters and weak-AC targets. On its legendary actions, it moves to maintain distance while using tail attacks and wing buffets to separate party members.
Environment & Setting
A vast swamp or marshland with deep water, submerged ruins, and thick vegetation providing three-quarter cover. Broken stone pillars, sunken temples, or submerged caves serve as lair features that leverage the dragon's superior swimming and breathing abilities.