Path of the Hellfire Engine

Barbarians who draw from the Path of the Hellfire Engine are sadistic monsters to all who know them, seen as artificial beasts and demons of the abyss. These masochists have been experimented on, and imbued with technology of the 9 hells. Those who walk this path have been imbued with traits of The Hellfire Engine, and can call upon those technologies to summon even greater strengths than any other, at the risk of their own life.

Embodiment of the Engine

Starting when you choose this path at 3rd level, when you enter your rage your body painfully transforms, revealing the power of the nine hells lurking within you. Until your rage ends, you may manifest one or more aspects of the artificial demonic form embedded inside you; the human Hellfire Engine. Each selected benefit has an associated ability and health cost that is specified in their description. When activating any of these abilities when you enter rage, you will take an amount of damage equal to the health cost (being in rage does not prevent or reduce any damage dealt by any of the following features, including the health cost of activating said features). Whenever an ability or feature forces you or an enemy to make a saving throw, the DC of that saving throw is 8 + your Str + your Prof unless otherwise specified. This will sometimes be referred to as your “barbarian DC.”

You may choose any of the following; you must choose at least one, and you may choose as many or all of them if you wish. When your rage ends, all of your activated features retreat back into your body, and each injury you received from activating said features mend instantly (this does not return any lost HP to you)

• Chainsword Eruption. Bursting from the flesh in your hands, you spontaneously grow chainsaw blades that painfully dig into your body. These “chainswords” deal 1d8 + str slashing damage on a hit, and count as longswords for the sake of calculating proficiency. You may use your bonus action to make a single attack with your off-hand Chainsword as long as you have taken the attack action on your turn. In addition: once per turn, when you hit an attack with one of your chainswords, you can rev the saw on the blade to deal increased damage to an enemy; you can choose to take a number of damage equal to your rage damage, dealing a number of D4’s to the target equal to the damage done to yourself. The health cost to activate this feature is 2d8 slashing damage.
• Hellsteel Plating. As you enter rage, your skin becomes the color of crimson chrome and the texture of metal, and your body becomes rigid. While raging, you may add half of your proficiency bonus to your unarmored defense ability (rounded up). In addition, as a reaction to being struck with a melee attack from an attacker that you can see within 5 feet of you, you may extend your plated skin into large spines, taking 1d4 piercing damage as your skin tears, that pierce the triggering creature; that creature has its movement speed set to 0 until they succeed on a dexterity saving throw as an action. While that creature’s movement speed is 0, they inhabit the same space as you and move with you. You may only have one creature bound with this feature at a time. The health cost to activate this feature is 2d4 piercing damage. 
• Brimstone Ignition. The joints in your hips and knees pop as the sound of an engine roars out from your thighs and calves. Exhaust pipes burst out of your heels and your legs begin to burn with the heat of a running combustion engine. While this feature is active, you have an additional 10 feet of movement each turn. In addition, you may flood your internal engine to explode with a burst of speed; as a reaction (which you may take without a triggering effect) you may move up to half your speed in a direction of your choice, only in a straight line. The exhaust pipes in your heels begin to glow with heat and your legs begin to burn, dealing 1d4 fire damage whenever you use this reaction. The health cost to activate this feature is 1d8 fire damage. 
• Maw of the Furnace. The bones in your jaw begin to crack and dislodge, as metal plates appear just outside where your cheeks meet your lips, and flames erupt from within your throat. As a bonus action on your turn, while this feature is active, you may exude hellfire from your maw. All creatures in a 15ft cone originating from you must make a dexterity save (DC equals 8 + your Str + your Prof) or take 2d6 fire damage, taking half as much on a success. You take 1d6 fire damage every time you activate this ability. The health cost to activate this feature is 1d6 piercing damage.

Hellfire Resonance

Beginning at 6th level, you can force your enemies to feel the pain of the hellfire. When you take damage from any features from this subclass, you can choose one creature within 30 feet of you that you can see. That creature must make a Constitution saving throw against your barbarian DC. On a failed save, the creature takes fire damage equal to half the amount you took. On a successful save, the creature takes no damage.

Tolerance; Ignition

At 10th level, you have become accustomed to the constant assault that your abilities wage on your body, giving you partial control of your pain tolerance. While raging, you may choose to become resistant to any instance of self-damage done by a path of the hellfire engine feature. Features that are worded as “equal to the damage done to yourself” only apply to the final total, which means this feature can reduce the effect of other features; use only when necessary.

Call of Hellfire

At 14th level, on any of your turns while you are raging, you can use a bonus action to overload one of your active hellfire engine features, fusing all of your power into a single devastating attack. You choose one of the following options:

• Chainsword Blast. You swing your chainsaw blades in a wide arc, sending a wave of hellfire towards your enemies. Each creature in a 15-foot cone must make a Dexterity saving throw against your barbarian DC or take 6d8 slashing damage and 6d4 fire damage on a failed save, or half as much damage on a successful one.
• Hellsteel Spike. You launch a massive spike of metal from your skin, impaling a creature within 60 feet of you that you can see. The creature must make a Strength saving throw against your barbarian DC or take 12d4 piercing damage and be grappled by the spike. The creature can use its action to make a Strength check against your barbarian DC, ending the grapple on a success. You can grapple only one creature at a time with this feature.
• Brimstone Rocket. You ignite your legs and propel yourself towards a creature within 120 feet of you that you can see. You must move in a straight line and you can fly over obstacles and creatures. When you reach the creature, you make a melee weapon attack with advantage. If the attack hits, you deal an extra 6d6 fire damage to the target and each creature within 10 feet of it must make a Dexterity saving throw against your barbarian DC or take 6d6 fire damage on a failed save, or half as much damage on a successful one.
• Maw of the Inferno. You open your mouth and unleash a torrent of hellfire that engulfs everything in its path. Each creature in a 60-foot line must make a Constitution saving throw against your barbarian DC or take 12d6 fire damage on a failed save, or half as much damage on a successful one.

You can use each of these abilities once per rage. After using this ability, upon exiting your rage, you take fire damage equal to your barbarian level multiplied by the amount of abilities used.