Path of the Spellgorger

Your rage stems from a deeply unnatural source.  You may have been the test subject for a terrible arcane experiment or have made a pact with an eldritch horror.  Regardless of the exact origin of your abilities, they inspire fear in those unlucky enough to see them firsthand.

Gnawing Arm

Starting at 3rd level, when you rage, one of your arms undergoes a revolting transformation.  It enlarges and becomes a scaled tentacle with a lamprey-like mouth at the end instead of a hand.  You cannot wield items with this appendage.

However, the arm-mouth is a powerful weapon that cuts through your foes’ flesh and psyches.  You can make unarmed strikes with it.  If you hit with it, you deal 1d10 + your Strength modifier piercing damage + 1d4 psychic damage. 

If you score a critical with your arm-mouth, you can immediately attempt to grapple the target as it latches onto your foe.

Spellgorge

Beginning at 6th level, your arm-mouth counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Once per rage, you can reroll a saving throw that you fail against a spell or magical effect (DM's discretion).  If you do so, you must use the new roll.

Once per rage, when you succeed on a saving throw against a spell or magical effect or when a spell attack misses you, you can use your reaction to consume the magic with your arm-mouth.  If it's a spell, you gain temporary hit points equal to twice the spell’s level (or 1, in the case of cantrips) + half your Barbarian level.   If's a magical effect (such as a dragon's breath or a medusa's gaze), you gain temporary hit points equal to the hit dice of the creature creating the effect.

Shred Psyche

At 10th level, your arm-mouth can overload a creature's mind.  When you score a critical hit with it, creatures with an Intelligence of 3 or greater must make an Intelligence saving throw against a DC of 8 + your proficiency bonus + your Constitution modifier.  Creatures that fail are stunned until the end of your next turn.

Maddening Howl

Starting at 14th level, you can cause all those around you to become temporarily insane.  As an action while you are raging, you and your arm-mouth can together unleash a mind-shattering scream.  Each creature that can hear the howl within 30 feet of you must make a Wisdom saving throw against a DC of 8 + your proficiency bonus + your Constitution modifier.  Creatures that fail take 6d6 + your Barbarian level psychic damage and suffer the effects of Confusion for 1 minute, while those who succeed take half as much damage and are not confused.  Confused targets can repeat the saving throw at the end of their turns, ending the effect on a success.  This is not a concentration effect.

Once you use this feature, you can’t use it again until you finish a long rest.