Living Construct. Acheron reduces damage taken by 25 (Increased by the number of Soul Charges of any element he has), regardless of the source of the damage. Additionally, he restores 22 (4d10) hit points at the beginning of each of his turns, increased to 33 (6d10) if he consumed a Soul Charge during his previous turn.

Machine Lord. Acheron enhances all constructs within 300 ft. of himself. All enhanced constructs gain a +2 to armor class, +1 to attack and damage rolls, an additional 100 max hit points, and an additional action each round. In addition, he gains an additional 25 max hit points for each enhanced construct. 

Soul Charge. Acheron has 6 Souls, each attuned to a different element, those being Dark (Necrotic), Light (Radiant), Fire (Fire), Water (Cold), Earth (Thunder), and Air (Lightning). Dealing damage to a creature steals a Soul Charge from them, which last until the end of Acheron's next long rest. The element of the Soul Charge is the attuned element of the creature he damages, with Aether, Nether, and Arcana attuned creatures giving a charge of each of their sub-elements. A Soul Charge can be consumed as part of an action or legendary action, giving access to that action's Enhanced form. Only one Soul Charge can be consumed per round.

Mechanical Arms. Acheron has four mechanical tendrils protruding from his back. Each tendril has 50 hit points, an AC of 15, and immunity to poison and psychic damage. Each tendril regains 10 hit points each round, even if fully destroyed, but cannot be used again until they reach 25 hit points minimum. Acheron gains a +15 to his walking speed for each active tendril and has a climbing speed equal to his walking speed as long as at least one tendril is active. These tendrils have a Strength score of 21 and count as one size larger for purposes of carrying capacity. Each one ends in a clawed hand, allowing them to use items.

Acheron can make two actions each round, choosing from the options detailed below, or one of the actions detailed in the Player's Handbook.

Claw (# of Active Mechanical Arms). Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 19 (4d6 + 5) slashing damage, which counts as magical for purposes of bypassing immunities and resistances. Enhanced: Each Claw attack deals an additional 14 (4d6) damage of the elemental type of the Soul Charge consumed.

Arclight Bolt (# of Active Mechanical Arms). Ranged Weapon Attack: +12 to hit, range 300 ft., 1 target. Hit: 11 (1d12 + 5) force damage. Enhanced: Doubles the number of attacks made.

Missile Storm (12). Ranged Weapon Attack: +12 to hit, range 120 ft., 1 target. Hit: 4 (1d4 + 2) piercing damage, which counts as magical for purposes of bypassing immunities and resistances. Enhanced: Fires off 6 more missiles.

Arclight Staff (2). Ranged Weapon Attack: +12 to hit, range 1200 ft., 1 target. Hit: 18 (2d12 + 5) force damage. Enhanced: Fires off three additional shots. Each consecutive hit after the first deals an additional (1d12) force damage.

Arclight Beam. Acheron fires concussive energy from a node in his chest in a 60 ft. line that is 5 ft. wide. All creatures in the area must make a DC 19 Dexterity saving throw, taking 35 (10d6) force damage on a failed save, or half as much on a success. Enhanced: The beam becomes 120 ft. long and 15 ft. wide and deals an additional 14 (4d6) force damage.

Soul Surge. Acheron releases elemental energy from a node in his chest in a 30 ft. cone. All creatures in the area must make a DC 19 Dexterity saving throw, taking (2d6) damage of each elemental type on a failed save, or half as much on a success. Enhanced: Each elemental type deals an additional (1d6) damage and ignores resistances, immunities, and absorptions.

Force Field. Acheron generates a shield of energy around him that lasts until the end of his next turn. The shield takes all damage before he does. Excess damage after the shield is destroyed is dealt to him. The shield has 60 hit points, increased by the number of Soul Charges he has. Enhanced: The shield has 120 hit points, increased by the number of Soul Charges he has. The shield lasts for 1 minute.

Repair. Acheron targets himself, or a construct he can see within 300 ft. of himself. The target regains 60 (10d10 + 5) hit points. Enhanced: The healing is increased by the number of Soul Charges he has, and all constructs within range benefit from the bonus healing.

Acheron has 5 legendary actions he can take each round and can choose from the options below.

Claw (Costs 1 Action). Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 19 (3d6 + 5) slashing damage, which counts as magical for purposes of bypassing immunities and resistances. Enhanced: Makes two claw attacks, each dealing an additional 7 (2d6) force damage.

Arclight Snare (Costs 3 Actions). Ranged Weapon Attack: +12 to hit, range 60 ft., 1 target. Hit: 16 (2d10 + 5) force damage. A creature his has its speed halved for 1 minute. Enhanced: The target takes an additional 33 (6d10) force damage and is pulled to 30 ft. from Acheron. The target cannot move further than 30 ft. from Acheron for 1 minute.

Restore Machine (Costs 5 Actions). Target a construct within 300 ft. that has fallen to 0 hit points within the past minute. That construct regains half its maximum hit points and moves to this point in the initiative order. (3/LR) Enhanced: The target instead regains its full maximum hit points.