Path of the Implacable Juggernaut

You have been subjected to a cruel ordeal that changed you forever. Though you appear normal most of the time, blasphemous magical experiments have altered your fundamental nature. The unleashed curiosity of a renegade artificer, master necromancer, demented cultist, or other practitioner of ungodly enchantments left a monstrous alter ego seething within you, ready to be unleashed. Your rage is not only the fury of battle, but a metamorphosis into a terrifying new body far different from the one you were born with.

The Form of the Juggernaut
If you choose this primal path, it is up to you and your DM to determine the appearance of the larger form you can take on when you rage. Remember that this form grants benefits like those of an enlarge/reduce spell’s increase in size (including doubling of your physical dimensions, an even greater increase in weight, advantage on Strength checks and saves, and enlarged weapons), and so the description of the form should account for these things. A warforged would probably shift and transform to become a larger and deadlier construct, while beings of flesh and blood are more likely to grow into some kind of biological horror. Your weapons may appear to become part of you or transform to resemble natural weapons such as horns, fangs, or claws. The details of your new form give you no additional abilities except for the existing alterations granted by the Transforming Rage feature. Some possible ideas
include:

• A many-armed titan with over-sized metallic claws on each hand.
• A batrachian aquatic hybrid with scales and amphibious features.
• A mummified undead colossus, wrapped in strips of ancient cloth that seem to move of their own accord.
• A berserk green-skinned goliath grown too muscular for the clothing that hangs from your frame in rags.
• A horned fiend with goat-like hooves and fiery red skin.
• A bulging, hunchbacked, flesh golem-like construct.
• A massively muscular, hirsute form that mixes your features with those of a large animal, like the hybrid form of a werebear or other lycanthrope.
• A Large-sized version of a frost giant or other giant subtype.
• A roughly-hewn statue of gray stone that makes a scraping sound when it moves its heavy limbs.
• A cyclopean, tentacled horror straight from the mad depths of Xoriat or the Far Realm.
• A massive warforged plated in thick armor and bristling with blades.

-From Ravenloft Players Compendium page: 30-31

Transforming Rage

Starting when you choose this path at 3rd level while raging you can use a bonus action to shift into a larger and more horrifying form. This effect lasts for 1 minute and does not end when your rage ends, though it does end early if you die or end it as a bonus action. When you use this feature, it grants you the following benefits: 

• Your size increases by one category.
• Your physical size doubles in all dimensions and your weight is multiplied by eight. If there isn't enough room, you attain the maximum possible size in the space available.
• Everything you are wearing and carrying changes size with you, though any item you drop returns to normal size at once. 
• While your weapons are enlarged, they deal an extra 1d4 damage. 
• Your jump distance doubles. 
• Your new appearance gives you advantage on all Charisma (Intimidate) checks, but disadvantage on all other Charisma checks.

Dense Form

At 3rd level, your altered form resists all attempts to move it or change it further. Whenever you make a saving throw against a spell or effect that would alter your form, push you, or cause you to fall prone, roll 1d8 and add the die to your saving throw total. At 17th level, you roll a d10 instead of a d8.

Relentless Rage

At 6th level, your inhuman form heals itself when it suffers a grievous blow. While raging, when you take damage that reduces you to half your hit points or fewer, as a reaction you can instantly regain hit points equal to twice your barbarian level. Once you use this feature, you cannot do so again until you finish a short or long rest.

Preternatural Vigor

At 10th level, your unnatural fortitude allows you to shrug off effects that would devastate or destroy a normal creature. You add the extra die from your Dense Form feature to all your Constitution saving throws and death saves, in addition to saving throws against the effects listed under that feature. If applying this bonus to a death saving throw increases the total to 20 or higher, you gain the benefits of rolling a 20 on the d20.

Swift Retribution

At 14th level, you instinctively lash out when struck. Whenever a creature within 5 feet inflicts damage on you, it provokes an opportunity attack from you. If the opportunity attack hits, you can move up to 10 feet as part of the same reaction without provoking further opportunity attacks.