Path of the World-Breaker

Through events, purposeful or not, you have been twisted by transmutation magics. Your fragile mortal psyche split in twixt, and a childlike and brutish alter-ego taking root in the recesses of your mind. This secondary persona is able to manifest itself in periods of great stress, anger, or otherwise strong emotion if you can not hold on to control. Those that lose themself to this emotion are put in the background of their subconscious as their alternative takes hold, their physical form replacing yours; a monster of fantastical physical power. With very little say or control over the instinctual and bullish nature of this persona, you can only hope it is able to differ friend from foe.

Manifestation of Fury

At 3rd level, when you enter a rage you are overwhelmed by a supernatural wrath, a fracture of your psyche that takes hold of your mind and painfully reshapes your physical form. You grow to size medium if not already, and account for one size larger when determining the weight you can push, pull, or carry. Any armor being worn, or clothes worn on the torso, are destroyed during this transformation.

While in this form you replace the unarmed strike for the Furious Strike, a blow dealing 1d8+STR in damage. You can make this strike as a bonus action as well. This improves to a 1d10 at 6th level, and a 1d12 at 14th level. You have disadvantage on Intelligence Saving Throws while in this form.
You lose all skill proficiencies in this form besides Athletics, Perception, and Intimidation. You gain said proficiencies while in that form if you do not have them otherwise. 

Avoid it as you may, your body adapts to this overwhelming Fury, you can rage an additional amount equal to 1/2 PB, to a minimum of 1 additional times, and become immune to poison.

Wrath of the Broken

You become more versatile with your increased strength, gaining the following features. 

Overhead Slam: As an action, you drive both fists into the ground in a furious smash, damaging and repelling those around you. All creatures within a 20ft radius of you take 2d8+STR damage, and must make a STR saving throw. The DC for this save is 10+STR+DEX. Failing this throw knocks the creature 10ft away from you and renders them prone. Can be done an amount equal to your proficiency bonus, regained on short rest. Damage increases from using d8 to d10 at 11th level. 

Inhuman Durability: You are able to heal all but the most grievous of wounds. As a bonus action you can heal yourself for 1d12+CON. This improves to 2d12 at 14th level. You can do this twice per short rest. 

Your unarmed strikes overcome resistance to non-magical bludgeoning damage.

 

The Monster Within

Savage Agility: You gain the ability to leap great bounds, you can jump a distance equal to your movement speed, costing you half movement to do so. 
Your overall movement speed increases by 10ft while in rage form.

Boulder Toss: Calling upon your Malice, you can rip a chunk of debris from your surroundings and launch it towards your foes, crushing those who do not flee (See Boulder Toss). 

They won't let me Die: If you are knocked to 0 hitpoints while not raging, and not killed outright, you must make a Constitution Saving Throw. The DC of this save is 10 + 1/2 your Barbarian Level. Succeeding this save will cause you to rage and enter your transformed state. You gain temporary hit points equal to your Barbarian Level + CON modifier. Upon leaving rage, you are left with 1 hit point.

Pinnacle of Malice

At 14th level, your transformation reaches a new peak, granting the following features.

Not so Gentle Giant: While in your rage form, your size increases large, and you develop resistance to all but Force, Radiant, and Psychic.

Thunderclap: Slamming your hands together, you create an explosion of concussive force is released, wreaking havoc upon all who face you. Can be done twice per short rest.
(See Thunder Clap)

Lethal Protector: If an ally within 15ft of you is subject to an attack, you can use your reaction to get in the way of the blow, taking the damage yourself. Can be done twice per short rest.

The other side of your psyche is beginning to effect your body further. Your aging slows dramatically, increasing your maximum lifespan to 5-7x your race's normal.