Path of Eldritch Slaughter

This barbarian has made untold sacrifice allowing themselves to be host for a fragment of a Great Old One's mind to grant them power indiscriminate and immense to conquer in the name of the Eldritch Gods.

Slaughter

When entering a rage the red mist clouds the Barbarians mind as fury fills their body. Only blood will follow this barbarian as they carve their way through battle indiscriminately killing all who dares oppose them. 

Upon raging the Barbarian has an extra plus 5 to hit and deals an extra d4 damage to a creature if they have killed in the last hour. For every creature killed the extra damage increases to the next largest die such as d4 to d6 and so on. However this extra damage decays every turn the barbarian goes without attacking a creature. If charmed they attack everything including the charmer and must do so till unconscious/dead or the caster is decapitated, as their mind is too fractured and are unable to tell friend from foe if magically afflicted.

Unfathomable Anger

Your rage knows now bounds and tears at the barriers between worlds calling forth untold horrors to aid you in your rampage of death. When raging sacrifice 5 hit points to call forth a small or medium aberration that is allied to only you but will leave when your rage ends. It is not under your command but will prioritise attacking those who are attacking you.

Cthulhuian Smite

Your wrath invokes a Great Old one to lend you a fraction of their power striking the first creature in your direct path  once every two of your turns attacking at the end of your turn. A blast of crackling purple energy rips through the sky creating a 10ft radius blast zone where all creature except from the barbarian take 4d6 psychic damage.

Ancient Roar

The barbarian screams in abyssal and all creatures unable to understand must role a d20 and suffer the effect for 1 hour. Great Old One warlocks are unaffected by the effects below but if they roll an even number they are charmed by the barbarian and are seen as allies by summoned aberrations and the Cthulhuian smite.

1 the creature's mind is obliterated, their INT and WIS drop to one, spellcasters lose their spells, paladins take the damage of their own smites, their holy symbols burn, they take 1d4 psychic damage

2-10 the creature is blinded, frightened, deafened, falls prone and takes 2d8 psychic damage

10-15 the creature is frightened and takes 1d6 psychic damage

15-20 the creature takes 1d4 psychic damage and has disadvantage on any INT and WIS checks and if they cast a spell the take half its damage

Invocation of Forbidden Knowledge

The barbarian must sacrifice a creature that they have befriended or have a familial relation to permanently imbue their weapon/armour with a fragment of a mind of a Great Old One. If it is a weapon the barbarian takes 1d4 psychic damage every day they do not kill 4 small creatures, 3 medium or 1 large creature (if gargantuan it staves off the damage for 8 days) and in return the weapon will have an extra 1d6 to hit and upon hitting roll a d4 and if greater than two the target is frightened, takes an extra 1d8 psychic damage and if a spell caster cannot cast spells above 2nd level until they sacrifice to the Great Old One that imbued the weapon. If armour it adds 5 AC and if a successful attack is made against the Barbarian the attacker sees the true form of the Great Old One within and must make a wisdom saving throw with a modifier of -5 and if unsuccessful their mind explodes and are reanimated for 1 hour with the same rules as the aberrations summoned during the Barbarian's rage.