Path of the Abomination

You are a corrupted being. In your body resides two persons, you and the beast inside you. This beast is a sentient and evil creature that seeks nothing but death and destruction that can be released by the barbarian or can force it's way out as it become stronger. 

 Unlike other barbarians, the Abomination isn't necessarily physically strong or efficient in battle, instead, the beast is the one who is capable in battle and the force of will of the host is what determines said strength. In that sense, the host is not exactly a barbarian, however, it can still gain some strength from the beast's influence. This is compensated by the sheer strength the barbarian gain while raging. The beast appearance and personality is determined by it's origin. It could be a demon possessing the character, a curse sent by a god, a magical or genetic mutation or even something else. Whatever it is, the abomination is a chaotic evil being that is barely sentient but that will become smarter and stronger as the character evolves and will evolve alongside they. In that perspective, role-playing the creature is usually made by the dungeon master, but the player can also do it as far as they are willing to role play it's wishes and it's personality, which will probably come into conflict with the host and the party. 

Transformation

Starting at level 3 you can release the  beast inside you while you rage. You can't rage without transforming from now on. When you transform you go through the following changes while raging:

Healing negation: You can't be healed by magical means while transformed. 

Ability scores: Str and Cha ability scores swift and Int becomes 6.

Type: Your type changes to a one that resembles your origin ( e.g. Fiend, monstrosity, undead)

Alignment: Your alignment become Chaotic Evil

Claw attack: You can make a claw attack that deal 1d8+Str mod slashing damage ( The damage increases to a 1d10 at level 6, 1d12 at level 10 and 2d6 at level 14). If you make 2 claw attacks against a creature and both hit, you can attempt to grapple the creature. 

Bite attack: You can make a bite a bite attack against a creature grappled by you as a bonus action. It deals 1d10+Str mod Piercing damage ( The damage increases to a 1d12 at level 6, to 2d8 at level 10 and 2d10 at level 14)

Temporary hit points: You gain 1d12 temp hp when you rage ( This increase to 2d12, 3d12 and 4d12 when you hit levels 6, 10 and 14 respectively.)

 Wen the rage ends roll a d20. On a 1-10 nothing happens, on a 11-19 you contract a short term madness. On a 20 you roll a second d20, on a 1-15 you gain a long term madness and on a 16-20 a permanent madness. 

Beast control

At level 6 you are able to attempt to control the monster inside you. Your unarmed strikes now deal 1d4 damage and they are considered a finesse weapon for you. Using a bonus action on you turn, while not raging, you can swap your STR with your CHA until the start of your next turn. On the beginning of your turn after you used this ability, you need to make a CON save DC 5 or enter a rage. While raging by this effect, you can't end the rage prematurely and you are hostile to every creature. The DC on this save increases by 1 each time you use this ability. The DC resets to 5 after a long rest

The beast's INT increases to 8.

 

Greater Beast

Beginning at 10th level, you can use your action to frighten someone with your menacing presence while tranformed. When you do so, choose one creature that you can see within 30 feet of you. If the creature can see or hear you, it must succeed on a Wisdom saving throw (DC equal to 8 + your proficiency bonus + your Strength modifier) or be frightened of you until the end of your next turn. On subsequent turns, you can use your action to extend the duration of this effect on the frightened creature until the end of your next turn. This effect ends if the creature ends its turn out of line of sight or more than 60 feet away from you.

If the creature succeeds on its saving throw, you can’t use this feature on that creature again for 24 hours.

Also, your size increases to by one category while you are transformed. 

The beast INT increases to 10. 

Corrupted body

At level 14th your beast influence became part of your body and changes you organism. You gain resistance to poison damage and advantage against being poisoned. While raging, you gain immunity to poison damage and to the poisoned condition. You also gain advantage on CON saving throws. 

The beast's INT increases to 12