Clockwork Laviathian

Legendary Resistance (3/Day). If the leviathan fails a saving throw, it can choose to succeed instead

Repair.  At the end of the Leviathian's every other turn, it is able to move gears and replace it's body. Able to heal itself for 1d100 plus 30. This also adds plus 2 to it's AC. 

Configure.  The creature is able to shift and move around it's body in order to change the position of it's tail and head and have them be at any end of the body, capable of freely moving the components in it's body at will. 

Mechanical Body.  Anyone who ends their turn on the body of the Leviathain must make a DC 24 dex saving throw or fall prone and be grappled by the gears of the creature. Depending on their position, they can either be attacked with the grind ability by the tail, or the slam attack. 

Broken Gears. Whenver the Levaithain takes damage of 75 or more within the same turn, it's gears begin to fall off and break. Causing it to lose 1 AC 

Multiattack. Clockwork Leviathian can make 2 Slam attacks, 1 tail attack, one clockwork throw or one breath attack. Grind is used to replace either the tail or the slam attack, depending on where the person is grappled. 

Grind. The Clockwork Leviathian grinds it's gears together on any creature that it has grappled. The target must make a DC 26 strength save, dealing 3d10 bludgeoning damage on a failed save and half the on a successful one. If the target succeeds, they  can break out of the grapple and be left on top of the leviathian. 

Gear Throwing. Melee or Ranged Weapon Attack: +17 to hit, reach 10 ft. or range 120 ft., one target. Hit: 36 (2d12 + 10) force damage. If the target is an object, it takes triple damage

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 16 (2d12 + 10) bludgeoning damage plus The target must make a DC 26 strength save, dealing 3d10 bludgeoning damage on a failed save and half the on a successful one. If the target succeeds, they  can break out of the grapple and be left on top of the leviathian. 

Slam. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 15 (4d10 + 10) bludgeoning damage On a hit the target must make a DC 25 strength save or be caught within the creatures cogs and suffer the grappled condition. The creature can make a DC 13 strength save in order to break free of the grapple. 

Breath Attack. (Recharge 2-3). The Leviathian fires a beam of light in a 150-foot line that is 10 feet wide. Each creature in the line must make a DC 26 Dexterity saving throw, taking 60 (11d10) radiant damage on a failed save, or half as much damage on a successful one. A creature reduced to 0 hit points by this beam is disintegrated, leaving behind anything it was wearing or carrying.

The Clockwork Laviathain does not get attacks of opportunity

 The Clockwork leviathan can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The leviathan regains spent legendary actions at the start of its turn.

Camouflage.  The Leviathian shifts and blends in with the surrounding area, giving anyone attempting to attack it has disadvantage. This affect lasts until the start of the laviathan's next turn. (Recharge 5-6)

Tail. The Leviathian can make a tail attack

Stunning Roar (Recharge 5–6). The clockwork leviathian emits a roar in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw. On a failed save, a creature takes 27 (5d10) psychic damage, drops everything it’s holding, and is stunned until the beginning of the Leviathain's next turn. On a successful save, a creature takes half as much damage. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. This can be made within camoflauge and those affected can make a DC 15 perception check in order to find out whether or not they are able to find the postion of the creature.