Clockwork Hydra

Immutable Form. The dragon is immune to any spell or effect that would alter its form.

Magic Resistance. The hydra has advantage on saving throws against spells and other magical effects. 

Multiple Heads. The hydra has six heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever the hydra takes 20 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.

At the end of its turn, it grows back a head for each destroyed one, unless it has taken 20 Acid or Lighting damage since its last turn. To regrow heads, the hydra must be within 60 ft of a Clockwork Production Machine. The hydra regains 11 (2d10) hit points for each regrown head. 

Reactive Heads. For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Unusual Nature. The dragon doesn't require, air, food, drink, or sleep. 

Multiattack. The hydra makes as many bite attacks as it has heads. Any of these can be replaced with a use of Constrict or Steam Hiss.  

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

Constrict. Melee Weapon Attack: +8 to hit, reach 10ft., one target. Hit: 8 (1d6 + 5) bludgeoning damage, and the target is restrained (DC 16). While the target is restrained, the head can't attack another target, and the target makes a Strength saving throw at the end of each of its turns, ending the effect on itself on a success, or taking 13 (3d8) bludgeoning damage on a failure. 

Steam Hiss. One creature within 60 feet of the hydra must make a Dexterity saving throw (DC 16), taking 12 (4d6) fire damage on a failure, or half on a success. 

Magnetic Shockwave (Recharge 5-6).  The hydra releases a wave of electromagnetism. Each creature within 60 ft of the hydra must make a Strength saving throw (DC 16) or take 12 (4d6) bludgeoning damage and be pulled 15 ft towards the hydra, taking half damage and not being pulled on a success. Creatures wearing metal armor or clothes have disadvantage on the save. Creatures who failed drop all the metal gear they were holding, which is stuck to the hydra and requires using an action to make a Strength check against the DC to recover them. 

Plasma Beam (Recharge 6). The hydra releases an energy laser in a 90 ft long, 10 ft wide line. Each creature in the area must make a Dexterity saving throw (DC 16), taking 6 (1d12) force damage for each head the hydra currently has, or half damage on a success. 

Consume Construct. The hydra consumes one Construct with at least 10 hit points that it can see within range, and regains a number of hit points equal to those the construct had before consumed. If the hydra gains at least 20 hit points from this feature and has less than 5 heads, it also regrows a head.