Ancient Black Dragon

A black dragon will not befriend the party, but it will accept a transaction. The setup is almost always a shared enemy: a hag coven encroaching on the swamp, a paladin order burning lizardfolk villages the dragon considers livestock, a rival dragon. The party brings a problem the dragon already wanted solved, and the dragon brings 367 HP of acid and Spellcasting to the field. Establish in negotiation that the dragon expects payment in addition to the shared cause, usually a magic item or a soul-debt cashed in later.

In combat, the dragon does not coordinate. It opens with Acid Breath whether or not the rogue is in the line, because the rogue can take it. After that, Fear on whichever enemy looks most likely to escape, since prisoners are tribute and tribute is profit. Rend triple-stacks on whichever creature is wearing the best armor, and once that creature is down, the dragon claims the corpse and its gear. The party can negotiate a share later or not at all.

Cloud of Insects, Pounce, and Frightful Presence still cycle as legendary actions, and the dragon will use them on hostile NPCs the party would rather take alive. This is the cost of the favor. If the party tries to redirect the dragon mid-fight, it ignores them and continues with what it decided was efficient. Once the named enemy is dead, the dragon takes its share of the spoils and leaves through the swamp without a goodbye. Anyone who tries to stop it makes the next session's adversary list.

Have the dragon write its terms on a strip of leather and hand it to the party before the fight. Players will haggle a written contract. They will not haggle a verbal one, and afterward they will swear the dragon cheated.

An ancient black dragon runs the encounter from a swamp it has owned for centuries. The lair matters as much as the dragon. Tannin-stained water, sucking mud, mosquito clouds, hanging moss, and the bones of every prior delegation should all be visible before the first save is called. By the time the party realizes the trees they are hiding behind grow on top of dead adventurers, the dragon already knows where they are. Passive Perception 26 plus Blindsight 60 means they did not sneak up on it.

Open with Acid Breath if it has charged. The 90-foot line, 10-foot wide, DC 22 Dex save, 67 acid on a fail will catch a marching column flat and drop a squishy in one hit. After the breath, fly. Walking speed is only 40 ft. but flying speed is 80 ft., and there is no reason for a Gargantuan dragon to stand in melee with a paladin when it can hover and use Spellcasting. Cast Fear at the cleric and the wizard. Both spells, breath, and Rend share Charisma DC 21, so the squishy who failed the breath save is also losing the Wisdom save against Fear. Fear breaks Concentration, so anything the wizard had up evaporates.

Use the legendary actions to deny recovery. Cloud of Insects on the cleric the round after they cast a heal, because the Disadvantage on Concentration saves means the next hit drops their bless or spirit guardians. Frightful Presence is a second use of Fear at no spell slot cost. Pounce repositions and gets a Rend off, useful when a fighter has finally cornered the dragon. Save Legendary Resistances for save-or-suck spells (Banishment, Hold Monster, Polymorph). Burn the first one early to teach the party not to bother.

Acid Arrow at-will and 1/Day Vitriolic Sphere are your boss-stage tools when the breath is not back. Vitriolic Sphere on a clustered party is brutal, and Create Undead in the days before the fight means the swamp is already populated with ghouls the dragon raised from those prior delegations. The dragon does not surrender. If reduced below half, it submerges (40 ft. swim, Amphibious) and stalks the party from below for the rest of the session.

Read out the Passive Perception number when the party first enters the swamp. Twenty-six is a wall, and they should feel it before any dice roll happens.