Path of the Shadowfiend

Barbarians that follow this path are figurative, and sometimes literal, demons on the battlefield. Their rage is fueled by the energy of the lower planes; the chaos of the Abyss, the flames of Gehenna or the primal, base sins of the Nine Hells just to name a few. They are able to draw on powerful fiendish abilities while raging and use the souls of their slain foes to increase the power of their attacks.

The more they draw on the negative energy that suffuses the lower planes and the more enemies they slay and absorb, the more these warriors come to resemble the denizens that reside in those dark realms. 

Shadowraze

When you take this subclass at 3rd level, you can empower your strikes with energy from the plane of eternal war. Whenever you hit with a melee weapon attack using strength, you deal an additional 2 weapon dice of necrotic damage. When you gain the Extra Attack feature, if you hit with two attack rolls in a row, you can make a third attack as a bonus action against a creature you can see within 30 ft. This attack is 3 weapon dice of necrotic damage as the power of Acheron allows you to cleave distant foes.

Also at 3rd level, the power of the lower planes infuses you and grants you an ability that activates while you are raging. You learn more abilities as you level up and can choose an additional ability when you reach 6th, 10th and 14th level in this class. You can change one ability for any other one that you know whenever you gain a level in this class.

Abyssal Hide

Your rage causes your skin to thicken and harden in places, making you appear discolored and slightly patch-work. While you are raging, you have resistance to cold, lightning and poison damage.

Chasme Feet

Your rage defies the very force of gravity, letting you run across surfaces as if they were flat terrain. While you are raging, you can climb vertical surfaces and upside-down across ceilings, without making an ability check, while leaving your hands free.

Defiant Rage

Your rage boils away the magic of shapechange spells and abilities. While you are raging, you cannot be unwillingly forced to change your shape by any spell, ability or effect.

Dimensional Lock

The fabric of the multiverse seems to thicken and solidify around you as you begin your rage, making the air around you shimmer like heat haze. While you are raging, creatures other than you and your allies cannot teleport to or from a space within 60ft of you. Any attempt to do so is wasted.

Impish Resistance

As you enter your rage, your skin becomes a hellish red and takes on a slight sheen. While you are raging you have advantage on saving throws against spells and other magical effects.

Massive Arms

Fiendish strength fills your arms as you enter your rage, increasing their bulk and length dramatically. While you are raging, your reach for melee attacks increases by 5ft and when you hit with a melee weapon attack using strength, you apply your strength modifier to the damage roll twice.

Merregon Instincts

Your rage enhances your instincts, especially those that call you to defend your allies and prove that you can withstand any attack. While you are raging, when an ally within 5ft of you is hit with an attack, you can use your reaction to take the damage for that attack instead of your ally.

Narzugon Hail of Strikes

Your combat ability and strike speed are boosted by your rage. While you are raging, whenever you take the Attack action, you can make three attacks instead of two.

Rampage

A feral thirst for killing seizes you whenever you use your rage, the sound of howling fills your ears when you kill something, demanding that you kill again. While you are raging, whenever you reduce a creature to 0 hit points with a melee weapon attack on your turn, you can use a bonus action to move up to half of your speed and make a single melee attack.

Serpentine Reaction

Your rage makes you hyper-aware of all enemies within your reach and your reflexes become flicker quick. While you are raging, you can take an extra Reaction each round, but this reaction can only be used to make an attack of opportunity.

Shadowform

As you enter your rage, the outline of your form becomes indistinct and you seem to flicker in place. While you are raging you can move through creatures and objects as if they are difficult terrain. If you end your turn inside an object you take 1d10 force damage.

Yugoloth Escape

The tricksome magic of Gehenna fills you as you enter your rage, strange blue runes etching themselves up the sides of your legs, your back and around your eyes. While you are raging, you can teleport up to 60ft to an unoccupied space you can see as a bonus action.

Necromastery

At 6th level, you learn to siphon the life force of your enemies as you deal the killing blow. Whenever you kill a creature, you gain 1 point of Necromastery. Whenever you deal necrotic damage, you deal additional damage equal to your Necromastery score. Your Necromastery score maximum is 2 x your barbarian level. Whenever you are reduced to 0 hit points, you lose half of your Necromastery points.

Requiem of Souls

At 10th level, as an action you can blast some of your stolen life energy into a wave of howling necrotic destruction. You lose half of your Necromastery points and deal necrotic damage to every enemy within 20ft of you equal to 2 x the number of Necromastery points that you lose. This ability triggers automatically when you are reduced to 0 hit points.

Negative Conduit

At 14th level, you open yourself to the plane of negative energy when you begin your rage, using its dark energy to shrug off fatal blows. But at a cost. 

While you are raging, you cannot be reduced below 1 hit point. All the damage that you take that would reduce you past this point accumulates, when your rage ends you take all of this accumulated damage as necrotic damage. If this damage is enough to instantly kill you, you die and any spell that would restore you to life requires 5 times as many material components to cast on you.