Brass Guardian

Transforming Limbs. This creature equips its sword, cannon, and shield by physically transforming its arm into each of them. 

Shield. For each shield this creature has equipped, it gains +2 to its AC. 

Programming. This creature cannot be threatened, persuaded, or otherwise convinced to perform any action which would go against its master's orders. 

Battle Damage. Each of this creature's body parts (head, torso, and each arm and leg) has its own separate pool of Hit Points (author's note: the 40hp listed in the stat block is assigned to each limb, not the creature itself. D&D Beyond doesn't let you specify that in the stat block). If the creature is targeted by an attack, the attacker must declare which part of the creature's body it is attacking. Attacks which may deal damage to multiple targets (such as the Fireball spell) may target multiple parts. Attacks or effects which require a Saving Throw will be rolled only once by the creature itself, and not for each individual part. If a part is reduced to 0hp, that part is considered "destroyed" and the following effects take place: 

Head: The head has been crushed and crumpled, bending the exhaust pipe out of shape. The Guardian can no longer vent its exhaust properly and must make a DC 10 Constitution Saving Throw at the start of each of its turns. If it fails, its speed is reduced to 0ft for that turn. 

Arm: The arm is completely detached from the body and the creature (naturally) may not use it to wield a weapon or shield. 

Torso: The engine has been badly damaged and the creature must make a DC 10 Constitution Saving Throw at the start of its turn or be unable to take actions for that turn. 

Leg: The leg is completely detached from the body and the creature’s movement speed is reduced by 5ft for each missing leg. 

Combat Phases. As this creature accrues damage, it enters three "phases" of combat, adapting to the situation at hand. The phases are detailed as follows: 

Phase One: At the start of combat, this creature wields its sword in one hand and shield in the other. 

Phase Two: After two of the creature's parts are reduced to half of their hit point maximum, or after one part is destroyed, the creature replaces its sword with its cannon. If its sword arm has been destroyed, its shield is replaced with its cannon. 

Phase Three: After two of the creature's parts are reduced to half of their hit point maximum and one part is destroyed, or after two parts are destroyed, the creature may now take two actions on each of its turns. The creature may also take the Mini Guardian reaction and the Change Weapon bonus action on any of its turns when possible. 

Death: The creature dies if three of its parts are destroyed. 

Sword. Melee Weapon Attack: +6 to hit, reach 5ft., 1 target. Hit: 14 (3d6 + 4) Slashing damage. 

Cannon. Ranged Weapon Attack: +3 to hit, range 60/120 ft., 1 target. Hit: 18 (4d6 + 4) Bludgeoning damage. 

Unarmed Strike. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 11 (2d6 + 4) Bludgeoning damage. 

Change Weapon. If this creature is in Phase 3 of its combat and has at least one working arm, it may use a bonus action to change one or both of its arms to either a sword, a shield, or a cannon. 

Mini Guardian. If this creature is in Phase 3 of its combat, it may use its Reaction to construct a miniature version of itself out of its own damaged parts. This Mini Guardian is jettisoned from within the Guardian's torso, and lands in an unoccupied space within 5ft. of the Guardian. 

Each Mini Guardian has 1hp, AC 10, and Speed 40ft. It may only use its action to attack the nearest enemy creature. Its sword is a Melee Weapon Attack and deals 3 (1d6 + 0) Slashing damage on each hit. Its size is Small, and it has the same immunities and resistances as the Guardian itself. All of its Ability Scores are 10, except for INT, WIS, and CHA which are 6.