Path of the Hellspawn

Barbarians that walk through the path of the hellspawn find a conduit for their rage in something darker than most barbarians, as their fury burns from the Lower Planes themselves, able to conjure claws, chains and lashes of flaming and fiendish might to draw their enemies closer and attack with hellish ferocity.

Perhaps you had a near death experience that barely took you to one of the lower planes, or perhaps you made a contract with a powerful fiend. Some tieflings find that this power has resided within themselves all along as a result of their infernal or abyssal heritage. Regardless of your reason however, one thing is certain: not even the hells match your wrath.

Fiendborn

Beginning when you first pick this path at 3rd level, the power of fiends courses through you. You learn to speak, read and write either Abyssal or Infernal (your choice), and you learn the Thaumaturgy cantrip if you don't know it already. Constitution is your spellcasting ability for this spell.

Additionally, whenever you make a Charisma ability check to interact with a fiend, you do so with advantage. 

Grasp of the Hellspawn

Also at 3rd level, your connection to the lower planes manifests when you rage. When you enter your rage, you manifest an extra appendage that lasts until your rage ends. This appendage can be a chain you are holding, an infernal lash that sprouts from your self, or even a floating and flaming claw.

As a bonus action, or as part of the bonus action required to rage, you can strike at one creature within 15 feet of you, forcing it to make a Strength saving throw (DC= 8 + your proficiency bonus +  your Strength modifier). On a failed save, it is pulled in a straight line towards you, 5 feet directly in front of you.

Razing Wrath

Starting at 6th level, your fury surges with the wrath of fiends. As an action, you can unleash the wrath of the lower planes, choosing one of the following effects:

• Creatures of your choice within 10 feet of you must succeed on a Dexterity saving throw against your Grasp of the Hellspawn save DC or take fire damage equal to a number of d8s equal to your proficiency bonus, or half as much on a successful save.
• One creature of your choice within 30 feet of you must succeed on a Wisdom saving throw against your Grasp of the Hellspawn save DC or become frightened of you for 1 minute. The creature can repeat the saving throw at the end of each of its turns.

You can also use this feature whenever you roll a critical hit on a weapon attack or whenever you use your rage feature (no action required). When you do, it doesn't expend a use. Otherwise, you can use this feature once, regaining the ability to do so again when you finish a short or long rest.

Fiendish Stroke

At 10th level, your connection to the lower planes grants you greater power. Your Grasp of the Hellspawn can now affect creatures up to 30 feet away rather than 15, and if they succeed on the saving throw, they are still pulled 5 feet closer to you. 

In addition, when a creature fails their saving throw against your Grasp of the Hellspawn, you can do one of the following:

• Make one weapon attack. This weapon attack scores a critical hit on a 19 or 20 on the d20.
• Reduce the creature's speed by half until the end of your next turn.

You can use this feature twice per rage, regaining one expended use when you reduce a creature to 0 hit points.

Form of the Hellspawn

By the time you've reached 14th level, you've learnt to tap into the full might of your dark powers. Whenever you rage, you may choose to fully invoke your power, gaining temporary hit points equal to your Barbarian level and the following benefits during the duration of your rage:

• You manifest two appendages for your Grasp of the Hellspawn feature. You can use a bonus action to command both of them. 
• You count as a Fiend as well as your normal creature type.
• You gain immunity to fire damage.
• Whenever you reduce a hostile creature to 0 hit points or a hostile creature is reduced to 0 hit points within 5 feet of you, you regain hit points equal to your Barbarian level + your Constitution modifier.
• Your weapon attacks deal an additional 1d6 fire damage.

You can use this feature once, regaining the ability to do so again when you finish a short or long rest, or by expending a use of your Rage feature.