Ancient Clockwork Dragon

Mechanical Vulnerability. Any character fighting the dragon can see that the complex arrangement of its workings gives it a unique vulnerability. Any character within 5 feet of the construct can use an action to attempt a DC 24 Dexterity check using jeweler’s tools or thieves’ tools. On a success, the dragon takes 45 damage as its components start to grind, and it is no longer able to recharge its Pellet breath.

Adamantine Build. The Dragon is immune to critical hits.

Fire Absorption. Whenever the dragon is subjected to Fire damage, it regains a number of Hit Points equal to the Fire damage dealt.

Immutable Form. The dragon can’t shape-shift.

Magic Resistance. The dragon has Advantage on saving throws against spells and other magical effects.

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

Multiattack. The dragon makes three Rend or Ignition shot attacks. It can replace one attack with a use of (A) Pellet Breath or (B) Grease Jet

Rend. Melee Attack Roll: +16, reach 15 ft. Hit: 18 (2d8 + 10) Slashing damage plus 10 (2d8) Force damage.

Ignition shot. Ranged Attack Roll: +10, range 80/320 ft. Hit: 12 (2d8 + 3) Fire damage

Pellet Breath (Recharge 5–6). Dexterity Saving Throw: DC 24, each creature in a 90-foot cone. Failure: 82 (15d10) Piercing damage. Success: Half damage.

Grease Jet. Dexterity Saving Throw: DC 24, each creature in a 30-foot Cone. Failure: each creature standing in its area must succeed on a Dexterity saving throw or have the prone condition. A creature that enters the area or ends its turn there must also succeed on that save or fall Prone and lasts until cleaned or burned away. A creature coated in grease that takes fire damage becomes Ignited, taking an additional 4d6 fire damage at the start of each of its turns. While ignited, the creature can't regain hit points by any means. This effect lasts for 1 minute or until the flames are extinguished with an action.

Null Wave (3/Day). Trigger: A creature the dragon can see within 120 feet casts a spell. Response: The dragon targets the caster with a pulse of entropic stillness. The caster must succeed on a DC 24 Constitution saving throw or the spell immediately fails and the spell slot is wasted. In addition the target will have one magical effect of the dragon’s choice immediately end.

Legendary Action Uses: 3. 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.

Jet Dash. The dragon propels itself up to 30 feet in a straight line without provoking opportunity attacks. If it passes through a creature’s space, that creature must succeed on a Dexterity Saving Throw: DC 24 or have the  prone condition.

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

Overheat (Costs 2 Actions). Until the end of the dragon’s next turn, it emits intense heat in a 15-foot emanation. Whenever the Emanation enters a creature’s space and whenever a creature enters the Emanation or ends its turn there, the creature must make a Constitution Saving Throw: DC 24. On a failed save, the creature takes 18 (4d8) Fire damage. On a successful save, the creature takes half as much damage. A creature makes this save only once per turn.