Path of the Devourer

A prevailing theory among even the most skilled shamans and planar biologists claims these barbarians, or Devourers, once came into contact with the lingering essence of Orcus or Hadar. Though some bear unsettling marks of such influence, they are neither vassals of the Demon Prince nor Star Spawn or Heralds of Hadar. Instead, they appear in places where they should not be: in verdant fields suddenly soured, lands on the cusp of famine, or at times as nature’s balancing after giving too much to humanity. For the Devourer, rage becomes voracity, gorging on bloodshed and the ebb of the dying’s vitality, all while spitting at Death.

Hanging on Hunger

Starting at 3rd level, you are Undead in addition to your existing creature types. You gain resistance to Cold and Necrotic damage. You no longer need to sleep or breathe, though you may choose to. When taking a long rest, you must spend at least 4 hours in a semiconscious, motionless state, during which you can still hear normally. However,  if 1 week passes without you raging and restoring Hit Points via Lifeblight, you must make a Wisdom saving throw (DC 10) each time you enter this state. On each success, the DC increases by 3. On a failure, you immediately enter a rage: you can’t distinguish friend from foe, are immune to charm and compulsion, and must attack the nearest creature. You can’t end this rage until you restore Hit Points through Lifeblight after doing its extra damage. The DC resets to 10 once this rage ends.

Lifeblight

During your rage state, while currently below maximum Hit Points, the first creature you hit on each of your turns with a weapon attack or unarmed strike takes an extra 1d4 damage, provided it has life essence (Determined by DM), and if you're under half that max, add your Constitution modifier (min of +1) to it. After doing this instance of extra damage, you heal equal to half its rolled total (round down), and that target can’t regain hit points until the end of your next turn. This extra damage is cold or necrotic when you gain this feature, and it increases by 1d4 at certain levels in this class, increasing to 2d4 at 9th level, 3d4 at 13th level, and 4d4 at 17th level.

If a creature dies (except those without life essence) within 30 feet of you, you can use your reaction to gain temporary hit points equal to half your Barbarian level. If you dealt the killing blow, you gain an amount equal to half your Barbarian level + your proficiency. 

Cold

The extra damage is Cold damage.

Necrotic

The extra damage is Necrotic damage

Forced Evolution

At 6th level, Temporary Hit Points granted by this subclass’s features can now stack, up to a maximum equal to twice your Barbarian level, and whenever a creature gains these Temporary Hit Points, the amount is increased by your Rage bonus. Additionally, as a bonus action, you can expend 1 Barbarian Hit Die and roll a number of d12s equal to half your proficiency bonus; you take Necrotic damage equal to their total. This damage ignores resistance, immunity, and any Temporary Hit Points you currently have. Then, you can either gain Temporary Hit Points equal to the damage you took for this bonus action, or have an ally within 30 feet gain half as much.

Ravening Brutality

At 10th level, while raging, your speed increases by 15 feet when moving closer to a living creature, and if you move 30 feet in a straight line before taking the Attack action, you can forgo Lifeblight on your first attack to apply the Topple mastery property to it (if it doesn't have it already). If a creature is knocked prone by this attack, you can immediately use your Bonus action to make a special melee spell attack against it, or if you take the Attack action against a currently prone creature, you can replace one of your attacks with it instead, provided you haven’t used Lifeblight this turn. You are proficient with it, and you add your Strength modifier to attack and damage rolls.

Oroboros’ Unending Hunt

At 14th level, the melee spell attack from Ravening Brutality now adds your Rage bonus to its damage roll if you’re below half your maximum Hit Points, and the extra damage from Lifeblight now treats immunity as resistance and treats Undead as living creatures. Additionally, when you rage, you gain a max healing tally equal to your Barbarian level + your Constitution modifier (min of 0). It accumulates an amount equal to Hit Points restored via Lifeblight, allowing you to do the following, provided that tally isn't 0;

• When making a weapon attack or unarmed strike that doesn’t benefit from Lifeblight, you can consume this tally to have this attack do additional Force damage equal to the consumed total. If this attack misses, you only deal half of that Force damage. The healing tally resets to 0, even on a miss.
• As a Bonus action, you can release this tally, choosing creatures (except yourself) up to your Constitution modifier (min of 1) within 25 feet of you. They heal equal to the released total, then the healing tally resets to 0.