Grave Knight

The archetypal Grave Knight commands the power of life and death to bolster their strength. They believe that strength need not disappear with one's life, and that strength can be borrowed from the dead to empower the living. Though not proficient in spellcasting, Grave Knights are versed enough in necromancy to utilize the strength of undead soldiers placed in their command. 

Undead Retainer

Beginning at 3rd level, you gain access to a graveyard demiplane and from it, you may summon and command a skeleton. The skeleton is summoned when choosing this martial archetype and remains at your side until it is destroyed, or until you dismiss it to the demiplane with a bonus action. If dismissed, the skeleton can be recalled with a bonus action, causing it to emerge from the ground in an unoccupied space within 5 feet of you. If destroyed, a new skeleton emerges from the ground in an unoccupied space within 5 feet of you at dusk. 

The skeleton is friendly to you and your allies and will follow commands given to it by you (no action required). The skeleton enters combat on its own initiative count and will attack any and all hostile targets, until commanded otherwise. 

At 15th level, your skeleton is laid to rest and you are granted command of a wraith in its place, following the same summoning rules. 

Siphoning Strikes

Starting at 7th level, once on each of your turns, when you hit a creature with a weapon attack, you may harness your necromantic energy and deal an extra 1d10 necrotic damage. 

In addition, as a bonus action, you may may channel your necromantic power into your weapon. The next time you hit a creature with this weapon, you regain hit points equal to the damage dealt to the creature. You may use this bonus action a number of times equal to half of your proficiency bonus, rounded up, per long rest.

Exoskeleton

At 10th level, you gain the ability to summon fragments of bone from your demiplane to intercept and soften incoming attacks. Your Armor Class increases by 2.

Reinforcements from the Grave

Starting at 15th level, in addition to your wraith, you may summon and command a bone naga from your demiplane. If your wraith is summoned when you reach 15th level, the bone naga is immediately summoned and remains at your side until it is destroyed, or until you dismiss it to a demiplane with a bonus action. If your wraith is not currently summoned when you reach 15th level, and if it has been dismissed, the bone naga can be recalled with a bonus action, causing it to emerge from the ground in an unoccupied space within 5 feet of you. If destroyed, a new bone naga emerges from the ground in an unoccupied space within 5 feet of you at dusk. 

The bone naga is friendly to you and your allies and will follow commands given to it by you (no action required). The bone naga enters combat on its own initiative count, though its vitality is bound to that of your wraith. Whenever one of your summons takes damage, the other takes the same amount of damage. Each instance of damage can only affect each of your summons once. When one of your summons is destroyed, the other is also destroyed regardless of its current hit point total. Likewise, when one of your summons is dismissed, the other is also dismissed, and when one is recalled the other is recalled with it. 

For the purposes of this feature, your bone naga was a spirit naga in life.

Shadow of the Reaper

At 18th level, you exude death. When a creature (other than your summons) enters your melee range for the first time on a turn, or starts its turn there, the creature takes necrotic damage equal to your Constitution modifier. This damage cannot be reduced in any way. Additionally, no creature other than you may regain hit points while within your melee range.