Symbiote Soul

Maybe you, or an ancestor, made a deal with a spirit that inhabits your brain with you. You both need each other to live. This spirit could be a long dead ancestor hellbent on avenging their name, could be a Quori spirit who seeks to cause destruction and ruin, but needs a host. 
While not necessarily evil, many symbiotes become evil after being corrupted by their spirit, but some, a rare few, can convince the spirit of the virtues of good. These two are the strongest of all, and to unlock the maximum potential of their power, one will have to cave, but who will cave first? The will power of a centuries old spirit, or a mortal of unknown magical potential?

In order to take this subclass you must be kalashtar lineage.

Expanded Spell List

you gain spells from the expanded spell list table, these are counted as sorcerer spells for you and do not count against your spells prepared.

Expanded spell list

1st level: Armour of Agathys, Cause fear

3rd level: Darkness, Enlarge/Reduce

5th level: Fear, Spirit Guardians

7th level: Greater Invisibility, Phantasmal Killer

9th level: Contagion, animate object

Bond from Within

As a bonus action, you allow the spirit to control you further, showing more of the form of the monsterous form they truly are. You gain the following benefits.

-As an action you can make a melee weapon attack roll against a creature within 5ft, on a hit, you deal 1d6+spell casting modifier. Additionally, the target must make a constitution saving throw against your spell save DC or be poisoned for 1 minute, they can repeat the saving throw at the end of each of their turns.

-You can add your proficiency bonus to the damage of spells.

-Whenever you roll a charisma(Intimidation) ability check, you roll with advantage. 

-You gain advantage on concentration saving throws.

-At the start of each of your turns you gain 1d6 + constitution temporary hit points at the start of each turn.

-At the end of your turns, while in this form, you must make a DC 9 + highest level spell slot used in the turn. On a failed save, you transform into a monster. See stat lock below, controlled by you and the DM, the DM will tell you what action to use that round, but you can attempt a DC 20 charisma saving throw to take a different action on the turn, in an attempt to convince the monster to stop. At the end of a turn where you are transformed you can attempt a DC 25 wisdom saving throw to regain control, the DC decreasing by 5 each round. While not all spirits are evil, most are, and will work to their own interests, usually their own freedom, and will take any action necessary, including harming party members or enemies. The monster uses your imitative, and if any attribute of your is higher, the monster will use that instead. This includes, but is not limited to: AC, HP, stats, Spell save DC, Familiars etc. Every turn that the monster is in control you can attempt to make a DC30 charisma saving throw to achieve cooperation, where you control the monster and gain certain abilities later on. You can lose cooperation only if you do something that the monster wouldn't like, or goes against their requests majorly. You do not lose cooperation when out of monster form. If you make a roll to achieve cooperation, you cannot make a wisdom saving throw to regain control, or a charisma saving throw to get it to do something else.

To end the transformation, you can use a bonus action while you have more than half your HP, otherwise, you must make a DC 15 wisdom saving throw to end the transformation. Once you've transformed into the monster that day, you must complete a short or long rest before you can transform into the symbiote, if you have cooperation, you can transform limitlessly.

https://www.dndbeyond.com/monsters/3999108-monster

Menace

Starting at 6th level, your connection to the symbiote grows, your minds becoming more and more similar. The DC to achieve cooperation falls to 25, and decreases to 20 by level 10. You can also choose one of the following benefits from the list below. 

-Melee Menace: You claw damage increases to 1d10, and whenever you attack with your claws you can attack twice instead of once. In addition, as long as you have cooperation, creature hit by your claws must make a constitution saving throw against your spell save DC or become poisoned for 1 minute and take and additional 1d10 poison damage, once and enemy has failed a saving throw, you cannot use this ability again until your next turn. The target can repeat the saving throw at the end of each of their turns.

-Mobile Menace: You gain an additional 10ft to your walking speed, you jumping distance is tripled, and you can take the dash action as a bonus action. Whenever you have cooperation, your symbiote is able to give you appendages that assist in more than walking, choose one of the following speeds to gain access to: 20ft flying, also, for every 1ft you fall, you can glide 2ft. 40ft swimming, you can also breathe underwater. 30ft climbing, and whenever you are on a wall you gain advantage on attack rolls.

-Magical Menace: Whenever you cast a spell you can choose to apply one of your metamagic options for no cost, you can only do this a number of times equal to your proficiency bonus and regain all expended uses on a long rest. In addition, whenever you have cooperation, you can increase your spell save DC and spell attack bonus by 1 and whenever you cast a transmutation spell, you can target an additional target.

You can swap your Menace on a long rest, or for 2 sorcery points and a bonus action. You only gain these benefits while in Symbiote Form

Heightened Senses

Starting at 14th level, you gain advantage on perception and investigation checks due to the symbiote growing stronger with you. Additionally whenever you have cooperation, your strength checks and saving throws have advantage.

Unison

You and your spirit have constant cooperation. Additionally, whenever in your symbiote form you can no longer lose control. Your symbiote form grows stronger as well, you gain access to another Menace and can swap them for free. You can also swap freely between the monster form and human form.