Mourning Blade Fighter Subclass

Level 3.

Grave Wound: Special melee weapon attack that changes your weapon damage to necrotic and does an additional 1d6 necrotic damage. 1d8 at level 7. 1d10 at level 10. Additionally, target must succeed on a constitution saving throw against your maneuver save DC or receive the Bone Chilled condition for two turns.

Bone Chilled - https://bg3.wiki/wiki/Bone_Chilled_(Condition)

Maneuver Save DC = 8 + Proficiency Bonus + Higher of Strength of Dexterity Modifier

Last Gasp: Once per long rest, when you are reduced to 0 HP but not killed outright, you instead drop to 1 HP and immediately gain temporary hit points equal to your Fighter level + Constitution modifier.

Cursed Resilience: You have Resistance to Necrotic Damage and Advantage on Saving Throws against the Frightened Condition.

Level 7.

Tenuous Resurrection: Once per turn when you reduce a creature to zero hit points with a melee weapon attack and necrotic damage, they'll be resurrected as a Blood Skeleton for three turns. Doesn't affect Constructs, Plants, Dragons, Ooze, or Elementals.

Blood Skeleton - https://bg3.wiki/wiki/Bloodmarrow_Skeleton

Gravepiercer: Your attacks ignore resistance and immunity to Necrotic Damage.

March of the Damned: Undead Allies within 9m have +3m Movement Speed.

You can cast Animate Dead at 3rd Level without expending a spell slot.

Level 10. 

One Foot in the Grave: You have a +1d4 bonus to melee weapon attacks and damage rolls against creatures with fewer than 50% of their hit points.

March of the Damned Improved: Undead Allies within 9m have +5m Movement Speed and Advantage on Attack Rolls.

When you cast Animate Dead without expending a spell slot, you can resurrect two Zombies or Skeletons instead of one.