Ancient Blue Dragon

An ancient blue on the party's side is a contract, not a friendship, and the contract is in the dragon's favor. The most plausible setups are a sworn rivalry against a worse enemy (a green dragon poaching its territory, a lich raising its hoard's former owners as undead), a debt of honor the dragon is paying off precisely once, or a bargain where the party is technically working for the dragon and only happens to share a target.

Run every interaction as a negotiation. The dragon casts Detect Magic at will and will inventory anything the party brings into its lair within minutes. It speaks Common and Draconic, and it will Send a follow-up message hours after the meeting to clarify the terms in a way that quietly favors itself. The party should leave every conversation feeling slightly worse off than when they arrived.

In combat, the dragon opens with Lightning Breath at maximum range and does not care which side of the line the party is standing on. Tell the players they have one round to clear the cone before the breath comes back online. Multiattack focuses whoever the dragon decides is the biggest immediate problem, usually a caster with visible spell components. Cloaked Flight is used to reposition for the next breath, never to save a party member. If a PC goes down in the area, the dragon does not loop back.

When the contracted task is finished, the dragon leaves the same hour. It takes payment in advance or it does not show.

Write the contract down on an index card before the session and read it out word for word. The dragon honors exactly what was written, nothing more.

An ancient blue dragon is a desert tyrant that fights from above, from below, and from inside an Invisibility spell. Don't run it like a meaner red. The kit is built around stand-off range, ambush, and a 120-foot Lightning Breath line that punishes the party for clustering up against the wind.

Open from invisibility if at all possible. The dragon casts Invisibility at will, and Cloaked Flight as a legendary action lets it both go invisible and fly half its 80-foot speed without spending a turn. Stage the first hit as a Lightning Breath line down a marching column. DC 23 Dex, 16d10 average 88, and the line is 10 feet wide and 120 feet long, so even a strung-out party loses two or three to one save. After the breath, settle into Multiattack: three Rends at +16 for roughly 29 each (87 on a full hit), with the option to swap one Rend for a level-3 Shatter on whoever clumped after the breath.

Legendary actions are how the dragon refuses to be pinned. Tail Swipe gives it a fourth Rend per round on someone else's turn. Sonic Boom is your reach when the breath is recharging and the melee characters are inside its space. Cloaked Flight is the panic button: if a paladin Smite is about to land, go invisible and reposition before the next hit. Burrow speed 40 ft. matters more than people remember; if the desert floor is sand, the dragon can vanish into it and re-emerge under a wagon.

Legendary Resistance is 4 per day, 5 in lair. Spend the first two on Hold Monster and Banishment without flinching. Save the last for whatever the party's actual win condition spell is. If Bloodied with no breath recharge in sight, cast Sending to a lieutenant, fly to ten thousand feet under Invisibility, and Scrying on the party that night to learn their names. The encounter does not end when initiative ends.

Have the dragon Send a single sentence to the party an hour before the fight. Something specific and personal that proves it has been watching. Then start initiative on its terms.