Path of Pain

Some barbarians fuel their rages with bloodlust, zealous devotion, or a connection to the primordial fury of raw elemental or magical power. Not so for a barbarian of the Path of Pain. To those on this path, the purest rage is attained through the experience of pain itself, the moments of clarity and strength derived from the adrenaline response driving their brutal attacks. The more damage a Pain barbarian takes, the more powerful they become, triggering new abilities the lower their hit points become. This drives many barbarians on this path to fight with absolute recklessness, embracing the damage they take in a perpetual game of chicken with their enemies' blades in order to experience as much cleansing pain as possible.

Reactive Rage

Starting when you choose this path at 3rd level, you can use the pain of your enemies' attacks to trigger your rage. When an enemy deals damage to you while you are not raging, you can choose to activate your rage as a reaction immediately after taking the damage. Any effects that trigger as part of the start of your rage, such as Tasha's optional Instinctive Pounce feature, can take place as part of this reaction, even if they would usually require a bonus action. Using this class feature still expends daily uses of your rage as if you had entered rage normally.

Adrenaline Burst

Beginning at 6th level, falling below 50% of your max HP while raging automatically triggers an Adrenaline Burst (no action required). When your Adrenaline Burst triggers, you can roll 1d6 at the beginning of your next turn and choose to either add the number rolled to the damage of your first successful attack that turn, or gain the number rolled in temporary HP (temporary hit points are tracked separately from max HP and don't count when determining class feature triggers reliant on proportions of max HP). Once Adrenaline Burst has been triggered and you have rolled your first Adrenaline Burst die, taking damage before your subsequent next turn allows you to gain the benefits of this feature on each of your following turns. Adrenaline Burst only ends once you don't take damage for a round. 

Additionally, the die rolled as part of this feature increases to 1d8 at level 10 and 1d10 at level 14.

Blood Price

Beginning at 10th level, you can use your Bonus Action to deal 1d12 damage to yourself. Immediately after doing so, and until the start of your next turn, you gain advantage on Strength checks and saving throws if you were not already raging, and if you are raging, your proficiency bonus is doubled for Strength checks and saves. You also deal extra damage equal to your Proficiency bonus on Strength-based melee attacks. The damage dealt to you by activating Blood Price bypasses rage resistances and is dealt to your hit points directly, ignoring temporary hit points. Blood Price cannot be used to trigger or maintain the effects of Adrenaline Burst, although it can sustain your rage if it would otherwise end. If your Blood Price damage puts you below 50% HP and would otherwise trigger Adrenaline Burst, it is instead triggered by the next instance of damage you take.

Once you have used this class feature, you cannot use it again until you regain all lost hit points.

Rejuvenating Agony

Starting at 14th level, when you are reduced to 0 hit points, instead of attempting to use your Relentless Rage class feature, you can choose to "fight for your life." While fighting for your life, you are still considered to have 0 hit points, although you do not fall unconscious. Your movement speed drops to 10 feet, you are immune to all damage, and you deal increased damage on Strength-based attacks equal to your Strength score. Your Rejuvenating Agony lasts for two rounds or until you kill an enemy. If you succeed in killing an enemy within those two rounds, you regain hit points equal to the amount of damage you dealt with the killing attack. If you do not succeed in killing an enemy in the allotted time, you fall unconscious at the end of the second turn and automatically fail your first two death saves. Healing effects cannot bring you out of Rejuvenating Agony, although they can restore you to consciousness in the event that you fail.