Adult Red Dragon

How to run it

Multiattack begins with Frightful Presence at DC 19, so the dragon can frighten the party and then make three attacks in the same action. A frightened character has disadvantage on attacks and cannot move closer, which is devastating against melee characters who need to close 60 feet.

Then three legendary actions a round on top. Tail Attack costs one, Wing Attack costs two and knocks everyone within 10 feet prone before the dragon flies away. The correct loop is breathe, claw, wing away, land somewhere else. A dragon that stays in one place is giving the party a free fight.

Legendary Resistance three times a day means the party's three best control spells do nothing. Expect that and let them burn through it, because watching a hold monster get shrugged off is part of what makes a dragon feel like a dragon.

Building the encounter

One adult red dragon is a deadly fight for a party of four at level 13 and a hard one at level 15. It is a campaign set piece, not a random encounter.

It needs a lair with vertical space and room to fly. 63 fire damage in a 60-foot cone at DC 21 will catch the whole party if they cluster, so give them cover to spread out behind or the fight ends on the first breath.

Budget for a long fight. 256 hit points and AC 19 means even an optimised party needs many rounds, and the dragon gets three legendary actions in every one of them.

What to pair it with

Nothing needs to fight beside it, and most things would just die to its own breath weapon. What it wants is a lair: chasms, lava, unstable ledges, a hoard it will not abandon.

If you want additional bodies, use them before the dragon arrives. A cult that delays the party and softens them up, then the dragon lands. Arriving second is far more frightening than being in the room from round one.

Reskins

The other chromatic dragons are the obvious reskin, and swapping fire for cold, acid or lightning changes the fight meaningfully because fire immunity is doing real work here.

Beyond dragons, the shape is a flying artillery platform with heavy melee, fear, and three extra actions a round. That describes very few things, which is why dragons feel unique. Reskinning this usually means building something else instead.

Commonly played wrong

Frightful Presence is part of Multiattack and costs nothing extra, so the dragon uses it every round it attacks. Many DMs treat it as a separate action and use it once. That halves the dragon's control.

Legendary actions are spent at the end of other creatures' turns, not on the dragon's own turn, and it gets three per round. A dragon that only acts on its initiative count is playing at roughly half strength.

Wing Attack lets the dragon fly half its speed afterward without provoking, which is how it disengages from a melee party every single round.

Compared to others at this CR

At CR 17 the adult red dragon is the standard against which other high-tier monsters get measured. Compared to its young version at CR 10, the difference is not the extra 78 hit points, it is legendary actions and Frightful Presence.

A young dragon takes turns like a monster. An adult acts four times a round and controls where the party can stand. That is the real gap between the tiers, and it is why a party that handled the young one comfortably can still lose to this.