Adult Black Dragon

A black dragon on the party's side has been bought, blackmailed, or briefly aligned by hate of a shared enemy. She does not like the players. She tolerates them because they're useful for the next hour. Set the deal in plain Common: one favor, one limit, one price, paid in something acidic to her interests (slaves to torment, a rival's hoard, the bones of a paladin who killed her brood).

In combat, run her selfish. Acid Breath catches whoever the party most wanted to interrogate, because that mouth is now a puddle. Rend goes on whichever PC has the prettiest armor, since she'll claim it after. Cloud of Insects as a legendary lands on the party's wizard if the wizard tried to give her orders. She follows the letter of the contract and nothing else, and she uses Detect Magic at-will to scan every chest and corpse on the field for things to pocket.

When the agreed work is done, she leaves through water. No goodbye, no spoils discussion, just gone. If anyone tries to stop her, Frightful Presence buys her one round and the swim speed buys the rest. Anyone who shorted her or claimed credit for her kill goes on the list. Black dragons keep lists.

Have her name the price out loud, in front of the whole party, before any swords come out. Half the table will hate the price. The other half will pay it. That argument is the scene.

An adult black dragon runs the encounter from a swamp. The water is the second monster. With a 40 ft. swim and Amphibious, this dragon will fight you from a flooded ruin or a sinkhole pool, and the moment things turn she submerges, regroups underwater, and surfaces somewhere the party can't reach with melee. Plan the terrain before you plan the turns.

Open with Acid Breath if positioning gives you the line. 12d8 acid in a 60-foot, 5-foot-wide line averages 54 on a failed DC 18 Dex save, which deletes a low-Con caster on a single bad roll. Recharge means you'll see it again, so don't waste it on a single grouped target if you can wait one round and clip three. Multiattack does three Rends at +11 for 17 average each (51 on a full hit), and you can swap one Rend for an Acid Arrow cast as part of the multiattack to chip a flying or hidden PC. Save the 1/Day Vitriolic Sphere for a clustered group; Speak with Dead is a roleplay button, not a fight tool.

Legendary actions own the encounter shape. Cloud of Insects on the cleric forces a DC 17 Dex save for 22 poison and Disadvantage on Concentration saves through their next turn, which is how you break a Bless or Spirit Guardians. Frightful Presence (a free Fear cast) at the end of the round denies movement to anyone who failed. Pounce gives you 20 ft. of movement plus a free Rend, perfect for repositioning to keep the breath line clean or to dive back into water.

Black dragons are cowards with long memories. Once Bloodied without a recharged breath, fly to deeper water or burrow into the muck and disappear. Three Legendary Resistances (four in lair) carry her through one Banishment or Hold Monster, but not two. Use the first save to win the fight, the second to escape.

Have her speak before she breathes. One mocking line in Common, naming a PC by something only the dragon should know, makes the swamp feel haunted before the dice come out.