Ancient Red Dragon

An ancient red dragon does not ally. It contracts. If your party has somehow leveraged one, the leverage is the entire scene: a stolen scale, a true name spoken aloud, an oath sworn on the soul of a younger wyrm the dragon still mourns, a Wish that the dragon owes a god the party serves. Without something that specific, the dragon walks. Even with it, the dragon resents the party and will look for the loophole the entire encounter.

Set the rules of the contract out loud at the table. The dragon will do one thing: burn the city, kill the lich, retrieve the artifact. It will not carry passengers, scout, take advice, or wait. If a player tries to ride it, the dragon hovers at 80 ft. and tells them to climb if they want, then takes off whether they're holding on or not. Treat every request beyond the contract as a chance for the dragon to extract another favor.

In combat the dragon plays for spectacle and loot. Fire Breath drops on the densest enemy cluster regardless of who the party wanted captured alive. Scorching Ray peels off whoever is wearing the most jewelry. The Pounce legendary is used to cross the battlefield and personally finish the named enemy commander, because the dragon wants the kill recorded. Commanding Presence will be used on the party's loudest player at least once, just to remind them whose favor they're cashing in.

When the contract is fulfilled, the dragon leaves the same turn. It takes the most valuable single object on the field as a tip and dares the party to object. Whoever spoke the true name has now made an enemy for life and should expect Scrying within the week.

Write the contract on an index card before the session and read it aloud when the dragon arrives. The dragon should feel like signing something, not making a friend.

An ancient red dragon is the top of the chromatic food chain and should be staged as such. This is a creature with 507 HP, AC 22, and Legendary Resistance four times per day (five in lair). The party will not "burst it down". They will survive it, push it off, or die. Set the table accordingly: the dragon has a name, a hoard, kobold staff, an opinion about the local kingdom, and a long memory.

The first round is Fire Breath. 26d6 fire (averaging 91), DC 24 Dex save, 90-foot cone, recharge 5 to 6. A failed save will outright drop most level 17 wizards and rogues. After it fires, the dragon climbs and circles. It can fly 80 ft. and Pounce as a legendary action grants another half-move plus a Rend, so even a "grounded" dragon is harder to lock down than the speed line suggests. Multiattack is three Rend attacks at +17 for 19 slashing plus 10 fire each, and one of those Rends can be swapped for a level-3 Scorching Ray. That swap is almost always correct against a clustered melee tank: six rays at +15 for 2d6 fire average more damage than a Rend and bypass shield bonuses.

Legendary actions are the encounter. Commanding Presence at the end of a healer's turn drops Command (level 2) on them with a DC 23 Wisdom save and steals an entire round of heals. Save Fiery Rays for the player who just landed a Counterspell. Pounce is your repositioning button when a wizard tries to drop Forcecage. Burn Legendary Resistances on Banishment, Hold Monster, and any control spell that would end the fight, not on Fireball.

Once it's Bloodied, fly straight up. The dragon does not die at the table unless the party has prepped specifically for this fight. Scrying lets it find them later, and the second encounter should be on the dragon's terms.

Name the dragon and have its kobolds use that name in dialogue an hour before the party sees it. The party should be afraid of a word, not a stat block.