Atlas Mk.I

Immutable. The Atlas Mk.I's Soul Engine prevents it from changing form via magical means.

Manned. The Atlas Mk.I is manned by three crewmembers, one of which is visible on the outside manning its Coaxial Chaingun, these members are not immune to line of sight attacks or spells such as Hypnotic Pattern. It can be assumed the crew breaks up as follows: Commander/Coaxial Chaingunner, Gunner/90mm Gunner, and Driver. The Driver sits in the most armored part of the chassis. 

Failures.
Optional (DMs discretion). The Atlas Mk.I requires many systems to operate efficiently, when hit by a Siege Weapon such as a cannon, or other similarly devastating magical effects such as Fireball, roll a d6 and consult this table.

d1. No Effect. The Atlas takes damage in a non-critical area and can operate as normal, but with cosmetic damage.

d2. Barrel Warping. The Atlas suffers damage to the barrel, the DM decides how severe. This can be rectified by replacing the barrel. The Atlas suffers disadvantage on every shot it takes with its 90mm afterwards.

d3. Hydraulic Failure. The Atlas suffers a hydraulic failure relating to its Legs. The Atlas moves at half speed until it spends an action (DC 14 Intelligence) to fix the problem.

d4. Magazine Blowout. The Atlas takes a direct hit to its magazine, setting off its payload. The crew may make a DC 15 Dexterity check to quench the flames. On a failure, the Atlas suffers 35d12 fire/bludgeoning damage from the resultant cookoff, crew rarely survives this encounter.

d5. Turret Ring Warping. Due to magical effect or physical damage, the Atlas can no longer turn it's turret ring, leaving the gun stuck in one direction and unable to change targets other than the last object or creature it shot. A DC 10 Dexterity save by the crew will rectify this fault.

d6. No Effect.  The Atlas takes damage in a non-critical area and can operate as normal, but with cosmetic damage.

90mm Cannon. Ranged Weapon Attack:+7, Range 120/500

The Atlas fires its main cannon at a target within range. The cannon then must cool for one turn and cannot be used. If the shell misses, all creatures within a 20ft radius of the target must make a DC 12 Dexterity saving throw or take 8d6 fire and piercing damage. On a successful save, the creature takes half damage. 

On Direct Hit. 
When the Atlas Mk.I hits its target with its 90mm cannon it instead deals 7d12 (45) fire/piercing damage to the target and does not generate a fireball. 

Stomp. Melee Weapon Attack:+4, Range 5ft

The Atlas steps on its foe. The target must make a DC10 Dexterity save to avoid being pinned (restrained).

Coaxial Chaingun. Ranged Weapon Attack:+3, Range 60/90

The Atlas fires its coaxial chaingun, firing three shots in quick succession, each shot dealing 1d10 damage. The target may use its reaction to attempt to dodge (DC 15 Dexterity) the slow moving projectiles, taking half damage on a successful save.