Avatar of Death, Rebalanced (5e Creature)
From D&D Wiki
Avatar of Death
Equal to the size of the creature that summoned it undead, neutral evil
Damage Resistances bludgeoning, piercing, and slashing from nonmagical attacks
Incorporeal Movement. The avatar can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
Legendary Resistance (1/Day). If the avatar fails a saving throw, it can choose to succeed instead.
Shared Power. The avatar gains the racial ability score increases of the creature that summoned it. Additionally, for every four total character levels the creature has, each of the avatar's ability scores increase by 1 (to a maximum of 22).
Turn Immunity. The avatar is immune to features that turn undead.
Unholy Power. Whenever the avatar deals damage to a creature, it also deals an additional number of d8s of necrotic damage equal to one less than the proficiency bonus of the creature that summoned it.
Unholy Resilience. The avatar is immune to any Channel Divinity option that would kill it without dealing damage.
Multiattack. If the creature that summoned the avatar has 11 or more levels, the avatar can make two reaping scythe attacks as an action.
Reaping Scythe. Melee Weapon Attack: +X (based on Charisma) to hit, reach 10 ft., one target. Hit: 7 (1d8 + 3) slashing damage.
The avatar of death is summoned by the Skull card from the deck of many things. This version of it attempts to make it legitimately deadly.