Path of the Monster

Your inner demons fuel your rage.  Most temper those demons by letting loose in combat, but inside you lies a monster that feeds off the confrontation, inching it ever closer to coming out, and taking control entirely.  It stirs within you always, and over time it comes ever closer to taking over all of your senses and wreaking havoc on every creature it can maul with reckless abandon - be they friend or foe.

Monstrous Form

At 3rd level your rage swells inside you, and your monster yearns to break free.  When you rage your size doubles in all dimensions, and your weight is multiplied by eight. This growth increases your size by one category - from Medium to Large, for example. If there isn't enough room for the you to double your size, you attain the maximum possible size in the space available. Melee weapons you wield, be they natural or crafted, do an additional d4 damage in addition to their normal damage while in Monstrous Form. Creatures grappled by you have disadvantage on Strength checks.

One Track Mind

At 6th level, while raging you have advantage on Wisdom and Charisma saving throws.

Run for your Life

At 10th level when you enter your rage, enemies who can see you and are within 15 feet of you must make a Wisdom saving throw against a DC that equals 8 + your proficiency bonus + your Strength modifier. Any enemies that fail this check gain the condition Frightened from you. The may try to repeat the save at the end of each of their turns. Once an enemy has made this save they are immune to Run for your Life for the rest of the combat.

When using the Dash action your movement speed increases by 10 feet while raging.

Rage Monster

At 14th level your monster comes ever closer to the surface and might take control. When you begin raging, and at the start of each of your turns while raging, you can relinquish control of your actions to the beast within until the start of your next turn. If you do, your Rage damage bonus is doubled and your attacks are critical hits on an unmodified roll of 19 or 20.  When you make the Attack action on your turn, you can make up to three attacks as part of that action, instead of one. While control is relinquished, you must use any action on your turn to take the Attack action, and each attack must be against the closest creature to you. If there are no targets within range, you instead must move your maximum movement towards the nearest creature and use your Reaction to ready the Attack action to attack any creature that comes within range. If there are multiple valid targets, you choose which one to attack.