Black Dragon Wyrmling

A wyrmling on the party's side is a stretch, and you should name the stretch openly. The most plausible version is a hatchling stolen from a clutch by a druid or ranger PC who is trying to raise it before its instincts harden. The dragon is not loyal. It tolerates the party because it is fed, warm, and not yet strong enough to leave. Every session, the GM rolls a private check to see whether something has tipped its loyalty.

In combat, the wyrmling fights from above or behind. With 60 ft. of fly speed and a 15-foot Acid Breath line, it picks one target the party has already engaged, breathes, and pulls back. It will not tank. Its 33 HP and AC 17 mean any dedicated attack drops it in two rounds, so a smart owner keeps it out of melee unless someone else is taking the heat. Two Rend attacks at +4 are a finisher, not an opener.

Use Amphibious for utility. The dragon scouts underwater, retrieves objects from flooded passages, and carries messages between an aquatic NPC and the party. It speaks only Draconic, so any conversation requires the spell, the language, or a translator the party trusts.

The dragon will leave eventually, and the leaving should hurt. Foreshadow it. The first session, it kills something it was told not to. The second, it lies. The third, it goes home to a parent who has finally found it. Whatever the players got from it should outlast the relationship.

A black dragon wyrmling is a swamp ambush with teeth. It is small enough to lair under a rotten dock or inside a half-flooded crypt, smart enough at Int 10 to set up the fight on its own ground, and fast enough in the water (40 ft. swim, Amphibious) to break off whenever the math turns against it. Run it as the first scaly enemy that actually thinks, not as a hatchling kobold with a breath weapon.

Open with Acid Breath if the wyrmling has line of sight to two PCs in a row. The 15-foot line, 5 ft. wide, DC 11 Dex save, 22 acid on a fail will likely down a wizard or rogue outright at low levels and cripple anyone who fails next to them. Open from cover when possible, since AC 17 with 33 HP will not survive a focused round of return fire. After the breath, fly to 60 ft. or dive into water and use the dragon's good Stealth to disengage and reposition for next round.

If the breath weapon is unavailable, the wyrmling commits to Multiattack only against an isolated target. Two Rend attacks at +4 average 7 each plus 2 acid, which is a real threat to a 1st or 2nd-level character but not enough to fight a full party in melee. The wyrmling is Acid immune, so it will gladly fight inside its own breath cloud and over acid pools it has been spitting into for years.

Wyrmlings are cowards with hoards. Once it drops below half HP, the dragon disengages, swims into deeper water, and screams for its mother if she exists in your campaign. If she does not, it simply vanishes and reappears two sessions later in a different fen, larger and angrier. Note who hurt it.

Set the encounter in three inches of black water and have the dragon breathe through the surface from below. Players who can't see the line until the save call land much harder.