Your rage has become unbounded, granting you special benefits as you become more desperate, but at a steep cost.

Upon raging, you gain a "Rage Total", which resets to 0 upon losing your rage. For every 5 raw damage to take from an attack, and every 4 damage that you deal during an attack, add 1 point to your rage total. Additionally, as a Bonus Action, you may spend an extra rage to add your barbarian level to your rage total OR spend one hit dice from your existing pool, roll it, and add that number. You may keep your rage total if you begin a new rage on the last turn of your current rage.

Your rage total, in conjunction with how much remaining health you have, determines what levels you can access. As your rage builds, you may gain the effects of each consecutive level, in addition to every previous level's. You may choose whether or not you want to activate each level as a bonus action upon meeting its' requirements. You lose all buffs and debuffs upon losing your rage, or being healed beyond the level requirements. You lose all debuffs upon losing your rage, unless stated otherwise.

Level 1: 3/4 health or lower. Rage Total > 15

Strength+2, even if this puts your total beyond 20. Speed + 10

Intelligence - 2

Level 2: 1/2 health or lower. Total > 25

You may use your bonus action to make take an extra melee attack action. Extra attack +1 if you are in a frenzied rage.

all attacks must be made recklessly,  Wisdom - 2

Level 3: 1/4 health or lower. Rage Total > 40

Extra attack + 1, You may use your constitution score for any Con. checks and saves, similar to the "Indomitable Might" class feature.

Disadvantage on all Intelligence/Wisdom checks and saving throws. Exhaustion +1 upon losing rage.

Level 4: 1/10 health or lower. Rage Total >  60

You gain resistance to all types of damage. Once per round upon hitting an enemy, you can force the target to make a Constitution saving throw against your contesting Strength roll, on a failed save, the attack automatically deals max weapon damage, including the additional damage dice granted by "Brutal Crit" if appropriate.

You are unable to interact with any creature, object, PC, or NPC, except to attack it. Additional exhaustion + 1 upon losing rage.

This was inspired by Mrpandorica's "Adrenaline Surge" feat, so go check their's out if you like the idea behind this, but not the stats.