Undying Will
At first level you gain the necrotic affinity as well as access to the following benefits.
Deathly Guise: You can use your bonus action to transform for a minute a number of times up to your proficiency bonus. Once per turn before making an attack you may attempt to unsettle your target with your haunting visage, the target must make a wisdom save against your Special DC granting you advantage on your first attack roll against them on a failure. While transformed via your deathly guise feature any damage you deal with an attack can be converted into necrotic damage and you may use your special modifier for attack and damage rolls.
Reapers Hour: Once per turn whenever you kill a creature you may gain temporary hit points equal to your proficiency bonus, if the creature died as a result of taking necrotic damage you may instead regain hit points equal to half your proficiency bonus rounded up. While transformed into your deathly guise you instead gain twice your proficiency bonus in temporary hit points or your full proficiency bonus in hit points.
Death's Cunning: You may use Wisdom in place of intelligence for calculating your special DC and attack bonus.
Not alive or dead...
At 6th level you gain the following features
Inured to death: You gain resistance to necrotic damage and your hit point maximum cannot be reduced. While transformed into your deathly guise you gain DR equal to your constitution modifier and half your proficiency bonus.
Bump in the night: You no longer need to sleep instead resting motionless but still fully conscious during rests, in addition you cannot be put to sleep by any means though you may still be knocked unconscious. You also gain darkvision out to a range of 60 feet and are unaffected by non-technique induced darkness.
But something in-between
At 11'th level you are granted more benefits befitting your undead nature
Walking Corpse: Your deathly guise brings forth the fear of death within all things. At the end of each of your turns each creature of your choice within 60 feet of you that can see you must make a wisdom saving throw gaining one rank of frightened on a failed save. Creatures will remain frightened of you in this way until they succeed the saving throw at the end of your turn removing one rank of frightened on a success or gaining an addition rank of frightened on a failure.
Just die already: Whenever you would take damage that reduces you to 0 hit points but does not outright kill you, you do not fall unconscious instead continuing to take turns as normal whilst also making death saves. You still suffer the effects of taking damage while at 0 hit points and if you would die you die. Your failed death saves are also reset whenever you succeed your third save, are stabilized, or healed. If you would be stabilized you fall unconscious.
Spectral motion: You have advantage on checks and saves made to resist being grappled or restrained, in addition whenever you take damage you may expend a ferocity die and roll it reducing the damage taken by that much, then gaining DR equal to half the value rolled until the start of your next turn. While under the effects of your deathly guise you also gain a flight speed equal to your walking speed, cannot fall, and cannot take fall damage. In addition whenever an effect would push you a number of feet you may choose to increase or decrease the amount of distance moved up to your movement speed without expending it.