Young Blue Dragon

A young blue dragon as an ally requires a transactional setup. Blues are Lawful Evil and they do not befriend, but they do strike bargains, and a young one is more biddable than an adult because it has not yet built the hoard that makes it independent. The party probably has something the dragon wants: a relic, a piece of territory, intelligence on a rival, the head of a sphinx that insulted it.

Make the contract specific and bounded. The dragon agrees to one engagement (burn the bandit camp, scout the canyon, scare off the rival caravan) and leaves the moment that engagement ends. In combat, run it greedy. Lightning Breath at DC 16 catches whichever cluster looks tactically convenient regardless of friendly fire on the party's hirelings, because the dragon does not consider hirelings real. Multiattack with three Rend at +9 stacks on whichever target has the most expensive armor, since gear becomes loot once the target is dead.

Out of combat the dragon is a Lawful negotiator. It honors the deal exactly as written and not one inch further. If the party tries to extend the contract on the road, the dragon names a new price, in writing, in Draconic. If they refuse to pay, it leaves. If they try to stiff it later, it remembers the names and faces and, with Common and 120 ft. of Darkvision, will find them.

Have the dragon name the price first, in Draconic, and make the player who speaks it translate aloud. The roleplay friction is the point.

A young blue dragon is a desert raider with a 60-foot lightning line and a 20-ft. burrow speed. Run it as an ambusher first and a melee bruiser second. The dragon swims through sand the way a fish moves through water, surfaces underneath the party's formation, fires the breath weapon down the line of march, and dives back under before the action economy has caught up.

Open with Lightning Breath if you can hit two PCs in the line. The 60-foot line is 5 ft. wide, DC 16 Dex save, 55 lightning on a fail. That single round will likely drop the wizard or rogue and severely wound anyone who failed beside them. The dragon then either burrows away to recharge or, if the breath is still cooking, takes flight with 80 ft. of fly and Multiattacks with three Rend at +9, 12 slashing plus 5 lightning per hit. With reach 10 ft. it threatens two squares and can move freely between them while the party scatters.

Lean on the dragon's senses for ambushes. Passive Perception 19, Darkvision 120, and Blindsight 30 mean it sees the party long before they see it. The cleanest opening scene is a caravan crossing flat sand, the dragon surfaces under the lead wagon, breathes once down the line, and vanishes. The fight begins on the dragon's terms and the party spends the first three rounds trying to find it. Lightning immunity means a wizard's bolt-style spells are wasted, which is worth telegraphing in advance with a clue (a scorched circle of glass, a witness who survived).

Young blues are smart and not heroic. Once Bloodied (76 HP), the dragon flies straight up to where the party cannot reach and harasses with Rend dives only when the breath weapon recharges. If the math gets worse, it leaves and comes back the next night. Sleep watches matter for a week after.

Describe the sand bulging upward before initiative is rolled. The party should know something is under the floor, just not exactly where the line of breath will go.