Many barbarians find themselves burdened by their bodies, wishing for something stronger and more efficient in a fight. Barbarians that think this way often walk the Path of the Monster, a path that usually involves making a dubious deal with a demonic or infernal being  to gain an extra arm. Depending on the details of the deal the arm can take on a demonic, alien or even humanoid appearance; but its strength will be true regardless, allowing the barbarian to swing an extra weapon or even grapple an opponent with their appendage.

Third Arm

Starting when you choose this subclass at 3rd level, once per short rest, you can use an action to sprout a third arm from your torso which lasts 1 hour before decaying and breaking off; you can also use a bonus action to shed the arm early. While active, the extra arm gives you a second bonus action that can only be made using the third arm and can include a third weapon attack, requiring the same conditions as wielding a second weapon. You can also tri-wield while holding one weapon without the light trait, or duel wield with a two-handed weapon and a light weapon. The extra arm can also move around your body using 5 feet of your movement, but it cannot leave your torso. The arm cannot wield a shield.

Weapons of War

Upon reaching level 6, whilst your third arm is active, you can now wield weapons of sizes a degree of magnitude higher than a simple two-handed weapon. In order to wield one of these martial weapons, you have to have one specially forged; although each one has a listed price (e.g. Siege Axe costing 150 gold), this doesn't account for the service cost for commissioning such a large weapon, only accounting for the raw materials to craft it. The value of the service cost is up to the Dungeon Master's discretion, but 100 gold is the approximate base price. Each weapon also has it's own mastery property, which you obtain if you have mastery with its listed 'base weapon'.

Siege Axe: This three-handed version of the great axe costs 150 gold and weighs 25 lbs; attacks with it deal 1d12 slashing damage plus 1d6 bludgeoning damage. This weapon has the heavy, reach and three-handed traits.
Roomsweep: Once per turn, if you hit a creature with a melee attack using this weapon, you can make another melee attack with it against a second and third creature within 5 ft. of the first that’s within your reach. On a hit, the second creature takes this weapon's damage, but without your ability modifier (unless the modifier is negative).

Siege Bow: This three-handed version of the longbow costs 250 gold and weighs 8 lbs; attacks with it deal 2d6 piercing damage. This weapon has the ammunition (range 300/1200), heavy and three-handed traits.
Cripple: If you hit a creature with a ranged attack roll using this weapon, they have halved walking speed and must use their bonus action in order to move and cannot dash until the start of your next turn.

Siege Crossbow: This three-handed version of the heavy crossbow costs 250 gold and weighs 65 lbs; attacks with it deal 2d8 piercing damage. This weapon has the ammunition (range 300/1200), heavy, loading and three-handed traits.
Skew: If you hit a creature with an attack roll using this weapon, you can choose to 'skew' them, causing any additional creatures behind them within the weapon's range to take the same amount of damage if your attack roll is also hits past their armour classes; any creatures that block the attack cause it to stop skewing.

Siege Glaive: This three-handed version of the glaive costs 100 gold and weighs 20 lbs; attacks with it deal 2d8 slashing damage. This weapon has the heavy, reach and three-handed traits, this weapon's reach trait increases its range by 10 feet instead of 5.
Carve: If your attack roll with this weapon misses a creature, you can still make a damage roll, dealing halved damage instead.

Siege Halberd: This three-handed version of the halberd costs 100 gold and weighs 20 lbs; attacks with it deal 2d8 slashing damage. This weapon has the heavy, reach and three-handed traits, this weapon's reach trait increases its range by 10 feet instead of 5.
Roomsweep: Once per turn, if you hit a creature with a melee attack using this weapon, you can make another melee attack with it against a second and third creature within 5 ft. of the first that’s within your reach. On a hit, the second creature takes this weapon's damage, but without your ability modifier (unless the modifier is negative).

Siege Maul: This three-handed version of the maul costs 50 gold and weighs 35 lbs; attacks with it deal 3d6 bludgeoning damage. This weapon has the heavy, reach and three-handed traits.
Slam: If you hit a creature with a melee attack roll using this weapon, you can force it and all creatures within a 5-foot radius of it to make a DC 8 + your strength modifier + your proficiency bonus Constitution saving throw, becoming prone on a failed save.

Siege Pike: This three-handed version of the pike costs 25 gold and weighs 65 lbs; attacks with it deal 2d8 piercing damage. This weapon has the heavy, reach and three-handed traits, this weapon's reach trait increases its range by 10 feet instead of 5.
Skew: If you hit a creature with an attack roll using this weapon, you can choose to 'skew' them, causing any additional creatures behind them within the weapon's range to take the same amount of damage if your attack roll is also hits past their armour classes; any creatures that block the attack cause it to stop skewing.

Siege Sword: This three-handed version of the great sword costs 250 gold and weighs 20 lbs; attacks with it deal 3d6 slashing damage. This weapon has the heavy, reach and three-handed traits.
Carve: If your attack roll with this weapon misses a creature, you can still make a damage roll, dealing halved damage instead.

 

Permanent Arm

Beginning at level 10, you can activate your arm indefinitely and as many times as you wish.

Fourth Arm

Once you reach level 14, you gain the ability to grow a fourth and final arm once per short rest, lasting one hour. It functions identically to your third arm, giving you a third bonus action.