Adult Adamantine Dragon

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Bite or Slowing Breath.

Rend. Melee Attack Roll: +12, reach 10 ft. Hit: 21 (3d8 + 7) Slashing damage.

Bite. Melee Attack Roll: +12, reach 10 ft. Hit: 21 (3d8 + 7) Piercing damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 19) and is Restrained until the Grapple ends

Slowing Breath. Constitution Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: The target cannot take Reactions; its Speed is halved; it can take either an Action or a Bonus Action on its turn, not both.

Magma Breath (Recharge 5–6). Strength Saving Throw: DC 19, each creature in a 60-foot Cone. Failure: 28 (8d6) Fire damage 28 (8d6) Bludgeoning damage. Success: Half damage.

Swallow. Strength Saving Throw: DC 19, one Large or smaller creature Grappled by the Dragon (it can have up to two creatures swallowed at a time). Failure: The target is swallowed by the dragon, and the Grappled condition ends.
A swallowed creature has the Blinded and Restrained conditions, has Total Cover against attacks and other effects outside of the dragon, and takes 5d6 Acid damage at the start of each of the dragon's turns.

If the dragon takes 30 damage or more on a single turn from a creature inside it, the dragon must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls in a space within 5 feet of the dragon and has the Prone condition. If the dragon dies, any swallowed creature can escape from the corpse using 20 feet of movement exiting Prone.

Legendary Action Uses: 3/day (4/day in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Heart of the Forge. Dexterity Saving Throw: DC 18, each creature in a 20-foot Emanation originating from the Dragon. Failure: 11 (3d6) Fire damage and the target has the [conditions] Frightened [/conditions] until the start of the Dragon's next turn.

Shower of Blades. Dexterity Saving Throw: DC 18, each creature in a 30-ft long, 5-ft wide Line. Failure: 10 (4d4) Slashing damage. Success: Half damage

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.

Cinderfall. (1/round) Constitution Saving Throw: DC 18, each creature in a 20-ft Sphere centered on a point the dragon can see within 90-feet of it. Failure: The target has the Poisoned condition until the end of its next turn. Failure OR Success: The area is Heavily Obscured until the end of the dragon's next turn.