Path of the Symbiote

The Symbiote is a sapient--not fully sentient--hybrid ooze/aberration that melds with your blood, bones, muscles, and nervous system.  The Symbiote entered the barbarian's body sometime before reaching 3rd level, it was merely dormant until you take this primal path. Magic that detects oozes, aberrations, or humanoids of your type will register you as any and all of those. The Symbiote is called an amplifier in psionic circles. The Symbiote--even after it's awakened when you take this subclass--is often slumbering, though when you go into a rage it is fully roused. How aware a Symbiote Barbarian is that another creature is living within them/merged with their mind and body varies greatly.

Partnered Rage

Choose Aberration or Ooze when you enter a rage.  Deal an extra 1d4 acid damage when you hit with your melee attacks if you chose Ooze.  Deal an extra 1d4 psychic damage when you hit with your melee attacks if you chose Aberration.  This extra damage increases when you reach certain levels in this class: 1d6 at 5th level, 1d8 at 11th level, 1d12 at 17th level.

Additionally, while raging, you are immune to the damage type of the bonus damage you deal, and you are resistant to the other type.  (If you chose Ooze, you are immune to acid damage and resistant to psychic damage while raging.  If you chose Aberration, you are immune to psychic damage and resistant to acid damage while raging.)

Second Layer Mind

You can speak telepathically to any creature you can see within 60 feet as long as that creature understands a language, this does not give the create the ability to respond telepathically. The Symbiote can also speak this way, though it does not truly understand language. The symbiote uses the same words, images, and sounds that its carrier uses to express anger to barrage their enemies with telepathic insults, mental screams, or psychic ravening gibberish; confounding and terrifying their enemies. When you are raging and deal damage to a creature within 60 feet, that creature has disadvantage on Constitution saving throws made to maintain concentration as a result of that damage.

 

Subconscious Charm

When you reach 6th level, the Symbiote within you grants you instinctive powers to influence others.  Gain a bonus to Charisma (Deception, Intimidation, and Persuasion) ability checks equal to your Constitution modifier (minimum 1).

Woven into Skull and Spine

When you reach 6th level, while raging, you have advantage on Intelligence, Wisdom, and Charisma saving throws as the Symbiote integrates itself more fully into your nervous system (or whatever you have that is most closely equivalent to a nervous system).

The Symbiote that Never Sleeps

When you reach 10th level, when a creature forces you to make a Wisdom saving throw or deals psychic damage to you, and that creature is on the same plane as you, you can use your reaction to have the creature take Psychic damage equal to 1d12 + your Constitution modifier + your Barbarian level.  If you are raging, that damage is doubled.  The creature must succeed on a Charisma saving throw with a DC of 8 + your proficiency bonus + your Constitution modifier or be incapacitated until the end of its next turn.  You can use this reaction a number of times equal to your proficiency bonus; you regain all uses at the end of a long rest.

Form of the Symbiote

When you reach 10th level, you can now squeeze through a space as narrow as one inch (though your gear will not be so malleable). Your speed increases by 5 feet as your new-found fluidity aids your movement. You also gain a climbing speed equal to your walking speed, and the ooze aspect of the Symbiote allows you to climb difficult to climb surfaces without needing to make an ability check, including vertical surfaces and ceilings.

 

Tendrils of the Passenger

Beginning at 14th level, when you enter a rage, a pair of pseudopods emerge from your body, typically from your armpits, though they can emerge from any part of your body. They do not impede your movement in any way, your other limbs can move through them freely. Whenever you take the attack action, gain two additional attacks, which must be made with the pseudopods (You can make all your attacks with the pseudopods if you want, but if you make 3 attacks, at least one must be a pseudopod, and if you make 4 attacks, at least 2 must be with the pseudopods). The pseudopods have a reach of 10, and attack using Strength. Their damage die is a 1d8 (magical bludgeoning damage), and they are affected by any and all benefits and bonus damage you gain from raging.

 

Puppeteer Parasite

When you reach 14th level, when you damage a creature with a pseudopod a little bit of the Symbiote is left behind within the target in the form of spores. If a non-construct creature that has been damaged by one of your pseudopods falls to 0 hit points, you can use your reaction to cause the Symbiote to briefly puppeteer them. The creature moves up to twice its speed and attacks a creature of your choice. If the creature has multiattack or extra attack, it uses it. The creature then collapses prone as normal for a creature that falls to 0 hit points. Regardless of whether you use the Symbiote's puppeteer ability or not, creatures damaged by your pseudopods within the last minute have disadvantage on death saving throws until that minute has elapsed. Constructs and creatures that don't leave a corpse when they die (shadows, wraith, some elementals) are immune to this ability.