Adult Blue Dragon

A blue dragon on the party's side is a transactional alliance, never a friendship. The dragon has decided that the party serves its current goal: a rival to remove, a caravan to scatter, a kobold settlement to reclaim. Make the goal explicit at the table, in dragon's own voice, before the first roll. Once the deal is named, the dragon holds to it with lawful precision and walks away the moment the terms are met.

In combat the dragon prefers to handle the air and let the party handle the ground. Lightning Breath drops on a clustered enemy line that the party would not want to charge anyway. Rend goes on whoever the party calls out by name. The dragon will not soak melee for them: if a fighter goes down, the dragon does not heal, does not interpose, and does not particularly care. It will, however, use Sending after the fight to brag, threaten, or gloat as the party chooses, since words are cheap.

Outside combat the dragon is your best information broker and your worst social liability. Scrying lets it answer questions the party cannot research. Invisibility lets it scout where the party cannot enter. But every favor it grants is logged. Anyone who slights it, withholds payment, or names it carelessly in a tavern earns a Sending in the night, in a voice the player will not forget.

Have the dragon name what it wants from this alliance in writing, on a card, before the first session it appears. The party should know exactly when the contract ends.

An adult blue dragon is a desert tyrant that fights from a position of total information control. Burrow 30, Fly 80, Cloaked Flight, Scrying once a day, Sending once a day. By the time the party meets it in person, the dragon has already known where they were for a week, and it has chosen the ground.

Open with Lightning Breath if it is available. 60 lightning in a 90-foot line, DC 19 Dex save, 5 feet wide. The line shape is the trick: place the dragon so the line catches two PCs minimum, ideally a caster and a melee. Recharge 5-6 means you should plan a second use by round three. Between breaths, Multiattack gives three Rend at +12 (21 average per Rend, with the lightning rider on each), and you can replace one Rend with Shatter to punish a clustered party for an extra 3d8 thunder.

Legendary actions are where the dragon controls the fight tempo. Cloaked Flight is the showpiece: cast Invisibility on itself and fly 40 feet, all at the end of a player's turn. The party loses target. They can ready an action, but the dragon is gone before they line up the shot. Sonic Boom (free Shatter) goes on a clustered group. Tail Swipe (free Rend) goes on whoever is bloodied. Save Cloaked Flight for the round after Lightning Breath fires, so the party cannot retaliate against a known position.

Blue dragons do not panic, but they do calculate. Once Bloodied with breath spent, the dragon turns Invisible via legendary action, burrows 30 feet down through unworked stone, and emerges somewhere the party cannot reach. They will not see it again until it has rebuilt its plan, possibly through Scrying followed by a Sending that says something cruel and specific.

Run the encounter in a place with sand or open dunes, not a tight cave. The dragon needs sky and the party needs the dread of being targetable from a thousand feet up.