Symbiotic Entity (Phase 1)

     In your travels you've found or encountered an otherworldly entity, either by stumbling upon a fresh entity, or by being chosen by it. Discuss with your DM if this is something you could've gotten it in your backstory. Use this as Phase 1 and talk to your DM about when you progress on to Phase 2. Upon being conjoined with the entity, you will gain boons and a flaw which allow you enhance your physique at the cost of having a roommate in your head at all times. The Entity is in a symbiotic relationship with you, Commensalism Relationship, and if it is ever removed forceful or willingly, it will seek the nearest host, or it will die, to continue on its motives and quests. 
          - The Entity cannot be removed willingly, unless you die as a result of falling to 0 Hit Points, or if magic is used like the spells Wish, Heal, or any cleansing spell of 6th level or higher. The Entity is attached to you through magic, so any spell used to read your thoughts will interpret your thoughts together, causing confusion to the caster. The Entity is tethered to your life force and is considered "you" for spells that prevent creatures from entering. (A Wizard casts Magic Hut specifying that you may enter, the Entity is allowed as well)
          - While the Entity has supplanted its sentience into your mind, you are at all times susceptible to its influence. Your DM may ask you to make a Mental saving throw, Intelligence, Wisdom, or Charisma. If you fail, your DM may have the Entity exude some of its influence on your mind or actions. (The DC of the saving throw will always be determined by the DM and is impacted by your surroundings or mental strain/stress)
          - This is Phase 1 of 3, and during this Phase, the Entity is going to protect you to preserve itself. Without you, the Entity is not likely to survive trying to find a new host. You will gain the two features "Not Yet" and "Symbiotic Weaponry".

     Not Yet: This feature allows the Entity to intervene with fate and save you. When reduced to 0 Hit Points, but not killed outright, the Entity shields you just enough to sustain life, dropping you to 1 Hit Point instead. Once this feature is used, you cannot use it again until you finish a long rest.

     Symbiotic Weaponry: This feature allows you to create a natural melee weapon which you gain proficiency with, as a bonus action. You can choose the form it takes each time you summon it and it statistics will match that form you chose.
          -The weapon can stay summoned up to 1 minute before it is reabsorbed back into your body.
               You can use the bonus action a number of times equal to your CON modifier per Long Rest (minimum of 1).
                     Moreover while the weapon is out, you are Vulnerable to Thunder Damage as the Entity is weak to loud sounds unless shielded fully inside you.