Young Black Dragon

A young black dragon as an ally is a brittle, transactional thing. Black dragons do not befriend anyone, they negotiate. The framings that work: the party has paid her in slaves of a hated rival's making, blackmailed her with a stolen egg, or convinced her that a shared enemy will eat her swamp next. The deal is one favor, one limit, one price. Make her name all three out loud.

In combat, run her selfish. Acid Breath catches whoever the party most wanted to interrogate, because the line was conveniently aimed there. Multiattack focuses three Rends on a single enemy until that enemy drops; corpses do not sue for back pay. She does not coordinate with the party's tactics. She does not protect the party's casters, and she leaves the round the contract clock runs out. The 80 ft. fly speed and Amphibious trait mean she can leave through any river or low cloud, and she does. Acid immunity also means she can drop her own breath on a melee scrum without much concern for any allies caught in the line.

Out of combat she is the worst houseguest in the campaign. She steals small magical items, lies about it when caught, and demands the party's loudest player apologize for breathing too loudly near her. She speaks Common and Draconic and uses both to insult the party in three languages a turn.

What she refuses: anything that endangers her hoard, any fight against another black dragon, any debt of honor that cannot be paid in coin or pain. Slight her at the closing meeting and she remembers the slight for the next thirty years.

Have her name the price out loud, in front of the whole party, before any swords come out. The argument the price starts is the scene.

A young black dragon runs the encounter from a swamp, a flooded ruin, or a sea cave it has claimed for the decade. With Amphibious, 40 ft. walk, 80 ft. fly, and 40 ft. swim, this dragon picks the terrain and the party fights on it. AC 18, 127 HP, Stealth +5, Acrobatics-grade Dex saves at +5; she is a real threat to a level-5 party and she knows it. Plan the water before you plan the turns.

Open with Acid Breath if positioning gives you the line. 14d6 acid in a 30-foot, 5-foot-wide line averages 49 on a failed DC 14 Dex save, which deletes a low-Con caster in one bad roll and seriously hurts on a passed save. The line is narrow, so the party's marching order does the work for you: anyone caught behind the fighter eats the same damage. Recharge means the breath returns on a 5-6, so do not waste it on a single grouped target if you can wait a round and clip three.

After the breath, switch to Multiattack: three Rends at +7 with reach 10 ft. for 12 average each (9 slashing, 3 acid), 36 average on a full hit. The reach matters; she can stand 10 ft. back from the front line and still attack everyone in melee range without exposing herself to opportunity attacks from the wizard. If the party tries to flank, fly straight up 80 ft. and let the rogue burn an arrow turn instead of getting another sneak attack. Blindsight 30 ft. and Darkvision 120 ft. mean Fog Cloud and Darkness do not help the party much.

Young blacks are spiteful but not suicidal. Once Bloodied with no Acid Breath ready, dive into the swamp and use the swim speed to vanish. Acid immunity means she can hide in her own breath puddle if she has to. She returns three nights later, with a plan and a grudge, and the next encounter is in the party's camp.

Have her speak one line in Common from the dark before the breath. Something cruel, something specific. The party should know the swamp is awake.