Amalyn Deathguard

Deathless Fortitude. If damage reduces the Deathguard to 0 (or less) Hit Points, it makes a Constitution saving throw with a DC of 5 + the overkill damage taken, unless the damage is radiant or from a critical hit. On a success, the Deathguard drops to 1 Hit Point instead.
Additionally, the first time the Deathguard would fail this saving throw or otherwise die (from radiant damage or a critical hit, for example), it can choose to drop to 1 HP instead, unless its body is annihilated.

Spellcasting. The Deathguard is able to cast spells using an action -- however, casting these spells costs HP instead of spell slots. For spells that gain additional effects based on level, the Deathguard counts as a 5th Level spellcaster. The Deathguard may not 'upcast' spells (ie: pay additional resources to gain greater effects), only spells that automatically gain effects from levels are upgraded (such as damaging cantrips).
The Deathguard does not require material components for its spells. If a spell requires something to have any effect (such as Animate Dead requiring a corpse), the Deathguard still requires that component. The Deathguard may not use Temporary HP to cast spells, with the exception of temporary HP gained from Unending Vigil.
The Deathguard uses its Wisdom as its spellcasting ability.

Cantrips (casting a cantrip costs no HP): Sapping Sting, Toll The Dead
1st Level Spells (costs 5HP per spellcast): Cause Fear, Inflict Wounds, Ray of Sickness
2nd Level Spells (costs 10HP per spellcast): Ray of Enfeeblement, Blindness/Deafness
3rd Level Spells (costs 15HP per spellcast): Bestow Curse, Animate Dead, Vampiric Touch

Unending Vigil. Once per battle, the Deathguard may gain 29 (3d10 + 12) temporary Hit Points. When it uses this ability, it also gains advantage on all saving throws until the end of its next turn. The Deathguard can use these Hit Points in order to cast spells.

Consume Minion. The Deathguard can use this action to sacrifice a friendly undead currently under its control. This heals the Deathguard for all of the remaining HP its minion had. The minion's corpse is mutilated in a manner that renders it unusable for further necromancy purposes.

Drain Soul. Once per battle, the Deathguard can use this action to make an unarmed strike against a target within 5ft of it. If the strike hits, the target must make a DC 16 Constitution Saving Throw or take 44 (4d10 + 20) necrotic damage (or half as much on a successful saving throw) as well as the damage from the unarmed strike. This damage ignores resistance and immunity to Necrotic damage and restores health to the Deathguard equal to the damage this attack dealt overall. The target, upon failing the saving throw, has disadvantage on Constitution and Wisdom saving throws until they complete a long rest.

Death March. Once per battle, the Deathguard can use this action to make all its minions act on the same turn as itself; the Deathguard and all minions ignore movement reduction from difficult terrain, instantly get up at no movement cost if they were prone (unless they are impeded from getting up by a condition such as grappling or being tied up -- this does not allow them to make checks to free themselves as a free action) and any fright or charm effects affecting them are outright ended. Additionally, the Deathguard may make a Cursed Longsword attack on a target within 5ft of itself when it uses this action. The next turn of all the Deathguard's minions (whether they acted before the Deathguard, or would have acted after) is skipped (they essentially used it ahead of time).

Double Strike. The Deathguard makes two Cursed Longsword attacks.

Cursed Longsword. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 12 (1d8 + 7) Slashing damage. (Versatile: # (1d10 + 7)). The target must also make a DC 14 CON Save or take an additional 5 (1d8) Necrotic damage.

Opportunity Attack. When an enemy leaves the reach of the Deathguard, it can make a melee attack against them.

Riposte. When an enemy misses a melee attack against the Deathguard, it can make a Cursed Longsword attack against them.

Death Shield. When an enemy successfully hits the Deathguard with an attack, it can give itself 9 (1d6 + 4) Temporary HP to absorb some of the attack's damage.

Parry. When an enemy successfuly hits the Deathguard with a melee attack, it can add +4 to its AC in order to attempt to block the attack.