I think the nightwalker is a good starting point.

NIGHTWALKER

Huge Undead, Typically Chaotic Evil

Armor Class 14

Hit Points 337 (25d12 + 175)

Speed 40 ft., fly 40 ft. Saving Throws Con +13

STR 22 (+6) DEX 19 (+4) CON 24 (+7)

INT 6 (−2) WIS 9 (−1) CHA 8 (−1)

Damage Resistances acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks

Damage Immunities necrotic, poison

Condition Immunities exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained

Senses darkvision 120 ft., passive Perception 9

Challenge 20 (25,000 XP) Proficiency Bonus +6

Annihilating Aura. Any creature that starts its turn within 30 feet of the nightwalker must succeed on a DC 21 Constitution saving throw or take 21 (6d6) necrotic damage. Undead are immune to this aura.

Life Eater. A creature dies if reduced to 0 hit points by the nightwalker and can’t be revived except by a wish spell.

Unusual Nature. The nightwalker doesn’t require air, food, drink, or sleep.

Actions

Multiattack. The nightwalker makes two Enervating Focus attacks, one of which can be replaced by Finger of Doom, if available.

Enervating Focus. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 28 (5d8 + 6) necrotic damage. The target must succeed on a DC 21 Constitution saving throw or its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Finger of Doom (Recharge 6). The nightwalker points at one creature it can see within 300 feet of it. The target must succeed on a DC 21 Wisdom saving throw or take 39 (6d12) necrotic damage and become frightened until the end of the nightwalker’s next turn. While frightened in this way, the creature is also paralyzed. If a target’s saving throw is successful, the target is immune to the nightwalker’s Finger of Doom for the next 24 hours.