You have discovered forgotten ways of integrating shadow energies into your fighting style, using your own life force to empower your blows and preying on the souls of your enemies to sustain yourself.

Negative Self

At 3rd level when you start tapping into shadow energies, your very soul is changed. You gain the following traits:

• Dusk Soul: you gain resistance to Necrotic damage, but spells that would make you regain hit points no longer affect you in any way. You still need the sustenance of food and water, and can still benefit from the use of Potion of Healing or any other non-spell effect that would make you regain hit points.
• Soul Eater: once per turn, when you hit an enemy with a weapon you are proficient with, you regain a number of hit points equal to your Fighter level.

Your aspect does not change much, but your physical colors takes on a more dull shade, save for your eyes, which seem to be slightly brighter than normal creatures of your race.

Dusk Infusion

At 3rd level, you learn how to infuse your attacks with shadow energies, increasing their deadliness at the cos of your own life force.
On your turn when using the Attack action and hitting an enemy, you can spend and roll a number of your hit dice up to half your proficiency bonus, you lose hit points equal to the number rolled and then add the same amount to the damage you inflict as Necrotic damage.

Dusk Apparition

At 7th level, when your Soul Eater feature would raise your hit points above your maximum, a Dusk Wisp is created and its hit points are equal to the excess from the Soul Eater feature. If a Dusk Wisp's hit points reach its maximum of 20, then a new Dusk Wisp is created. You can have a number of active Wisps equal to your WIS modifier (minimum of 1).
A Dusk Wisp is a creature with the same statistics and qualities of a Will-o'-Wisp, but with the following modifications:

• the Wisp cannot attack;
• the Wisp initial Hit Points equal to the exceeding healing you could not benefit from;
• the Wisp AC is 15;
• the Wisp has flying speed of 25 feet;

The wisp appears in an unoccupied space in the range of 10 feet from you, and you can use your reaction to move it up to 25 feet in any direction.
As long as a Dusk Wisp is in your view, you add to it any excess hit points from your Soul Eater feature.
As a Bonus Action in your turn, you can decide to Devour one of your active Dusk Wisps. The closest wisp will immediately teleport to you, and you regain a number of hit points equal to half the remaining amount (rounded down) in the wisp, consuming it in the process.

Improved Infusion

At 10th level you get more accustomed to the stress your shadow arts impose on you. When you use your Dusk Infusion feature, you can now use Hit Dice up to your full Proficiency Bonus.

Dusk Appearance

At 10th level, you learn to draw on your shadow arts to instill pure terror in the hearts of your enemies. 
You gain proficiency in the Intimidation skill.  If you are already proficient in that skill, you add double your proficiency bonus to checks you make with it, and can use your Wisdom modifier instead of Charisma for this skill, as you impose your will on the mind of your target.
Additionally, while you are at or below half health, whenever a humanoid with challenge rating equal to or less than your Wisdom modifier (minimum 1/4) starts its turn able to see you, if you can see it, you can force it to succeed on a Wisdom saving throw or be frightened of you until it can't see you or you can't see it.  The DC for this saving throw is equal to your passive Intimidation score - 2.

Undying Hunger

At 15th level, whenever damage would put you at 0 hit points, you can use your Reaction to Devour one of your Dusk Wisps or perform one weapon attack to any enemy within range. This attack will benefit from your Soul Eater feature, but cannot be used with the Dusk Infusion feature.
Additionally, whenever you roll initiative at the start of a combat and have no Dusk Wisp available, you can decide to lose 5 hit points and generate a Dusk Wisp with 15 hit points.

Soul Mastery

At 18th level, whenever you attack with a weapon you are proficient into, you now recover hit points equal to your Fighter level plus your prficiency bonus. You also regain the full amount hit points left in your Wisp when Devouring one.
Additionally, when using your Dusk Infusion feature, you can decide to maximise the hit point loss and bonus Necrotic damage from a number of Hit Dice used for the activation up to your CON modifier each long rest.