Ancient Bronze Dragon

A bronze dragon flying on the party's side is what happens when someone has spent a campaign earning it. Bronzes are coast-keepers, oath-witnesses, judges of unjust kings. They show up because a sworn debt has come due or because the party is moving against something the dragon already wanted dead. Make the why explicit before the dragon arrives, because the bronze will say it out loud either way.

In combat the bronze opens with intelligence. Detect Thoughts (1/Day) at the start of the encounter to identify the leader and the deceiver, then a Guiding Bolt at level 2 to mark that creature for the party's strikers. Once melee is joined, Lightning Breath in a 120-foot line carves the field; place it so the party isn't in the cone, since they will not appreciate 82 lightning damage from their ally. Repulsion Breath is the rescue tool: when a PC is being grappled or surrounded, the bronze can shove the threats 60 feet away and prone them.

The bronze does not take orders. It coordinates. If a player tries to dictate targets, the dragon hears them out and then does what justice requires. It will refuse to kill prisoners, refuse to torch a town to flush out a foe, and refuse to keep fighting after the agreed objective is complete. Control Water (1/Day) is on the table for non-combat: flooding a fortress, parting a river, calming a storm at sea. Save Scrying for the next session prep, where the dragon promises to look in on a missing NPC.

Have the dragon swear something specific before it leaves. A sentence the party can call back on later, when they need help again and need to remind it what was promised.

A hostile bronze dragon does not start with claws out. Bronzes are Lawful Good metallics who interrogate before they execute, so the encounter usually opens with a conversation: a wrong answer, a false oath, a relic the party shouldn't be carrying. Run the first scene at parley distance and let the party choose whether they're really doing this. Once the dragon decides they are, the gloves come off and the encounter shifts to a CR 22 air-superiority kill box.

Open from above. The bronze has 80 ft. of fly speed, 444 HP, and Lightning Breath in a 120-foot line at DC 23 Dex for 82 average damage. Lead with the breath the moment the party is bunched, then drop into Multiattack range for three Rend at +16 (27 each, 81 on a full hit). Multiattack lets you replace one Rend with Repulsion Breath, which is your get-off-me button: DC 23 Strength or every melee in a 30-foot cone gets shoved 60 feet and dropped Prone. Use it the round a paladin closes with Smites prepared.

Legendary actions tighten the loop. Pounce gives a free move plus a Rend on someone else's turn, which keeps the dragon repositioning between breath weapon recharges. Thunderclap is a 20-foot sphere centered anywhere within 120 feet at DC 22 Con: 13 thunder and Deafened. Drop it on whichever cluster is healing back up. Save Legendary Resistance (4/Day, 5/Day in Lair) for the spells that would actually end the fight, like Banishment or Hold Monster, and don't burn one on a Frightened save.

The bronze is Lawful Good, so it offers terms when Bloodied if the party hasn't done something unforgivable. Have it land on a stone outcrop, fold its wings, and ask one question. If the answer is right, the fight ends. If it's wrong, the dragon lifts off and finishes the job. Players remember dragons that gave them an out more than dragons that just hit them.