Blightheart

Undead Fortitude. If damage reduces the blightheart to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the blightheart drops to 1 hit point instead.

Blighted Life. The blightheart is driven by a dark facsimile of life that seeks to grow.  Not corrupt or harm, just grow.  A blightheart does not need to eat, drink, or sleep.  When it touches a creature, it is infected with the blightshade, a magical illness that will eventually pass this form of life onto that creature.  The touched creature must succeed on a DC11 Constitution saving throw to resist the infection.  If a creature dies while afflicted, it is transformed into a blightheart, increasing its strength, dexterity, and constitution scores by 1 and decreasing its intelligence, wisdom, and charisma by 2.  The creature gains immunity to the poisoned condition, immunity to poison damage, and vulnerability to radiant damage.

Fist. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.

Shadetouch (Recharge 5-6). Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (2d4+2) bludgeoning damage plus 8 (4d4) necrotic damage.  If this connects, the creature must succeed on a DC11 Constitution saving throw or be infected with blightshade.  The blightheart heals for half the necrotic damage dealt, with any healing done over its hit point maximum converting to temporary hit points.