Anima Umbra

Anima Umbra is an sentient chaotic neutral being which feeds on the souls of truly sentient beings. Created by unknown means and unable to take on an physical humanlike or animallike apperance they are forced to endlessly seek for a host to inhabit. When awakened and attuned to a Host it levels with them and has the same level at his host (when it's host levels up it also levels up). While unable to take part in a fight as a companion of sorts, it can take on the vague apperance and properties of a melee weapon you are proficient with(the player defines it's physical properties). The being is able to levitate up to 7ft. in the air and when in weapon form gains the thrown property. If Anima Umbra is unawakened it takes on the appearence of a normal longsword.
The Anima Umbra can be awakened by harvesting 5 souls whose are then consumed. Anima Umbra has no own senses but can use the ones of his host and is able to communicate telepathically with the him. Umbra uses the main language of his host and if the host doesn't know any languages umbra teaches them common.  When not in combat it can take on the apperance of any clothing item you want or just stay in weapon form.  Every midnight Umbra consumes 1d4 souls. The being can't be away from his host for long. If more than 3000ft. away from it's host it consumes it's stored souls with an increased rate (2d8 per day). 

"Your Soul is mine"

Lifesteal (Awakened)
When hitting a target the Anima Umbra takes a little taste of the targets soul. When dealing more than half the max amount of damage possible (e.g. for a shortsword which has a d6 it would be 4, 5, or 6) you heal a amount of health depending on the level ((the amount of damage done by the throw / 4) d4 | d6 lvl 5 |  d8 lvl 10 | d10 lvl 15 | d20 lvl 20 (e.g. character lvl 3 with a greataxe throws a 11 on the d12 heals 11/4 d4 = 2d4 hp) but min 1. 

Devour of Souls (Awakened)
Umbra can trap a max amount of 50 souls. Souls are gained by delivering the killing strike to an truly sentient opponent (no sentient robot or other being where a "soul" wouldn't make sense)  or by using an action to suck all the souls in a 50ft. radius out of dead bodies (for this to work the corpse cannot be older than 3 days) and the souls count as a trapped souls. When a soul is devoured by the anima umbra there is no way other than wish to resurrect them. Depending on how many souls are stored the umbra gains a glowing red stream throughout it's body.

Soul devouring: At level 3 umbra gains the ability to heal it's user with the souls stored. If at least 5 souls are stored you can use a bonus action to devour up to 20 souls (5|10|15|20) and heal d4 | d8 lvl 10 | d10 lvl 15 | d20 lvl 20 up to the number of souls eaten / 5. This ability can also be used outside of battle.
Necrotic embalment: Starting at level 5 the host gains the ability to use 5 souls to coat the weapon with black flames for 1 Minute. This adds 1d4 | 2d4 lvl 5 | 2d6 lvl 10 | 3d8 lvl 15 |  3d10 lvl 20 Necrotic damage. This ability can be used up to the amount of times equal to the hosts profficiency bonus . 
Undying: At level 10 umbra gains the ability to revive their host when they're downed at the cost of 25 souls. This ability can be used once per battle.
True Form: At level 15 umbra gains the ability to morph into their true self. When having a grand soul stored (a soul of a particulary strong opponent) that soul can be used to turn umbra and the host into the moust powerful version posible. The host takes on a creepier appearence, burning bright eyes and wings with 30 ft. movement in the air. Any attack made by the host cannot miss. The host gains full health and 20 temporary hitpoints as well as doubled movement speed. In addition the host uses souls instead of spellslots (calculated by level of spell = cost of souls). The host also gains true vision and is uneffected by negative status effects. This state lasts for five minutes and can only be activated in the most extreme circumstances. 

Soulcontrol: When an enemy picks up umbra on his free will he has to make an dc 13 wisdom save or be charmed by umbra. The charmed target can re
Soulsense: Umbra can sense sentient souls in a 20ft. radius when there are no walls or other obstacles in between. If there's enough time umbra can warn his host from possible enemies. 
Soulless: If Umbra is unable to consume souls on midnight, it tries to take control of his host and slay anything in the area. The host has to make a wisdom saving throw and beat a 18 or be charmed until 1d4 souls have been consumed by the symbiote. If the host takes damage during this effect they can do the saving throw again but the consumed souls stay the same.

 Unbreakable: Anima Umbra is not killable by normal means. Only one way is currently known which is starvation. If the Anima Umbra cannot consume a soul for 7 days it falls into a deep slumber to prevent starvation. When in this state the Anima Umbra counts as unawakened and can only be reawakened by harvesting 5 souls. If Umbra is somehow unable to fall into a slumber after 7 days of starvation it will die.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: By a creature of non lawful alignment., Healing, Combat, Sentient, Versatile, Sap