Arsenal

Phase One: Barrier Drones. The arsenal has three barrier drones that float 10 feet above it and project protective fields onto it. A barrier drone has an AC of 15 and 25 hit points. If it is forced to make a saving throw, treat its ability scores as 10 (+0). If a barrier drone is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails. While at least one barrier drone is intact, the arsenal has resistance to all damage. When the arsenal would take damage, one of the barrier drones can intensify the protective field to reduce the damage to 0. Until the start of the arsenal’s next turn, attacks against this barrier drone have advantage and it cannot intensify the field again. If the arsenal is reduced to fewer than 240 hit points, the power source for the barrier drones is destroyed and any remaining barrier drones immediately fall to 0 hit points.

Phase Two: Defense Protocols. If all three barrier drones are destroyed, the arsenal reveals four shield generators, one in each wheel. It can now use its Elemental Defense Protocol Legendary action. Each generator has 20 AC, 20 hit points, and resistance to all damage. If it is forced to make a saving throw, treat its ability scores as 10 (+0). If a generator is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails. Destroying a generator causes it to explode and destroy its corresponding wheel, reducing the arsenal’s speed by 10 feet, as well as preventing the arsenal from using the corresponding defense protocol. If the arsenal is reduced to fewer than 160 hit points, it retracts any remaining generators. After doing so, it can no longer use its Elemental Defense Protocol legendary action and its remaining wheels cannot be targeted.

Phase Three: Mythic Drive. If the arsenal has all four of its shield generators destroyed or is reduced to fewer than 160 hit points, it activates its Mythic Drive and gains the ability to take Mythic Actions.

Phase Four: Overcharge. If the arsenal is reduced fewer than 80 hit points, it will use its Overcharge Shot as its action on every turn until combat ends.

Constructed Nature. The arsenal doesn’t require air, food, drink, or sleep. Instead of benefitting from short or long rests, the arsenal can use its auto-repair unit, which is detailed in its description.

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

Legendary Resistance (1/phase). If the arsenal fails a saving throw, it can choose to succeed instead. It can use this trait one time in each of its four combat phases detailed above.

Multiattack. The arsenal makes four attacks with its machine gun and uses its Charged Shot if it can.

Machine Gun. Ranged Weapon Attack: +10 to hit, range 80/240 ft., one target. Hit: 13 (2d8 + 4) piercing damage.

Charged Shot (Recharge 5-6). The arsenal readies one of the following attacks with its main cannon. Until the start of the arsenal’s next turn, the cannon can be targeted by attacks and has 16 AC. If the cannon is hit by an attack, the arsenal does not take damage, but it must succeed on a Constitution saving throw with a DC equal to the damage dealt. If it fails, it is stunned until the start of its next turn. At the start of the arsenal’s next turn, it fires its charged shot unless it was stunned.

Homing Laser. The arsenal locks onto a creature with a red targeting laser. So long as the target is within 60 feet of the arsenal when it fires the shot, the target takes 35 (10d4 + 10) force damage.

Pulse Blast. The arsenal illuminates a 120-foot long, 5-foot wide line in front of it with light. At the start of the arsenal’s next turn, each creature within that line must make a DC 18 Dexterity saving throw or take 35 (7d10) force damage. A creature that succeeds on the save can step to the side of the line or else it is subjected to the effects of a failed save.

Radiant Beam. Ranged Weapon Attack: +10 to hit, range 100/300 ft., one target, chosen when it fires the shot. Hit: 31 (6d8 + 4) radiant damage.

Overcharged Shot. The arsenal can only use this action if it has fewer than 80 hit points. The arsenal begins charging an incredibly powerful shot in its main cannon. While charging this shot, the arsenal cannot take any bonus actions, reactions, lair actions, or legendary actions and it is immune to being stunned. At the start of its next turn, the arsenal unleashes a 120-foot cone of energy. Each creature within that cone must make a DC 22 Dexterity saving throw, taking 165 (30d10) force damage on a failed save, or half as much damage on a success. The only way to prevent it from firing this shot is to destroy it before its next turn.

Sensors. The arsenal can make a Wisdom Perception check as a bonus action.

Pot Shot. When a creature moves while within 10 feet of the arsenal, the arsenal can use its reaction to make a machine gun attack against the creature.

The arsenal 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 arsenal regains spent legendary actions at the start of its turn.

Attack. The arsenal makes an attack with its machine gun.

Elemental Defense Protocol (Costs 2 Actions). The arsenal activates one of the shield generators within its wheels to grant itself a damage immunity until its next turn, provided the corresponding generator is intact. The wheels correspond to the following immunities:

Front right: Cold; Back right: Fire; Back left; Lightning; Front left: Thunder

Full Speed Ahead (Costs 2 Actions). The arsenal moves up to its walking speed in a straight line and can move through the space of any creature smaller than Huge. The first time it enters a creature’s space during this move, the creature must make a DC 22 Strength saving throw or be pushed ahead of the arsenal for the rest of this move. On a successful save a creature can choose to be move to the side of the arsenal or to be pushed ahead. If a creature fails the save by 5 or more, it is also knocked prone and takes 18 (3d6+8) bludgeoning damage, or 29 (6d6+8) bludgeoning damage if it was already prone..

If the arsenal’s Mythic Drive trait is activated, it can use the options below as legendary actions.

Hover Jets. The arsenal gains a flying speed of 30 feet and the ability to hover until the end of its next turn.

Physical Defense Protocol. The arsenal gains resistance to nonmagical bludgeoning, piercing, and slashing damage until the start of its next turn.

Suppressing Fire. The arsenal makes two attacks with its machine gun against a single target or three attacks against three separate targets that are all within 10 feet of each other.

Supercharged Shot (Costs 2 Actions). This mythic action can only be used if the arsenal readied a Charged Shot on its previous turn. The arsenal supercharges the shot it is readying. The arsenal’s cannon has resistance to all damage while supercharging a shot. Radiant Beam becomes Antimatter Beam, Homing Laser becomes Obliterating Laser, and Pulse Blast becomes Pulse Cannon.

Antimatter Beam. Ranged Weapon Attack: +10 to hit, range 120/360 ft., one target, chosen when it fires the shot. Hit: 58 (12d8 + 4) necrotic damage.

Pulse Cannon. The arsenal illuminates a 120-foot long, 10-foot wide line in front of it with light. At the start of the arsenal’s next turn, each creature within that line must make a DC 21 Dexterity saving throw, taking 67 (15d8) force damage on a failed save or half as much damage on a successful one. 

Obliterating Laser. The arsenal locks onto a creature with a green targeting laser. So long as the target is within 120 feet of the arsenal when it fires the shot, the target must make a DC 18 Dexterity saving throw, taking 70 (20d4 + 20) force damage on a failed save, or half as much damage on a successful one.