Avatar of Death (Vecna Variant)

Avoidance. If the avatar is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

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.

Turn Immunity. The avatar is immune to features that turn Undead.

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

Multiattack. The avatar uses Cackle of Death. It then makes one Drain Vitality attack (if available) and one Spectral Scythe attack. Alternatively, the avatar makes two Spectral Scythe attacks.

Spectral Scythe. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 30 (4d12 + 4) force damage. A creature killed by this damage can't be restored to life. 

Cackle of Death. The avatar's skeletal jaws let forth a horrifying cackle. Each creature within 60 feet of the avatar, that can hear it, must make a DC 16 Wisdom saving throw or be frightened for 1 minute. If a creature fails by 5 or more it is also stunned for the duration. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the avatar's Cackle of Death for the next 24 hours.

Drain Vitality (Recharge 5-6). Melee or Ranged Spell Attack: +8 to hit, reach 5 ft. or 60 ft., one creature. Hit: 22 (4d8 + 4) necrotic damage and the creature's Constitution score is reduced by 1d6. The target dies if this reduces its Constitution to 0. Otherwise, the reduction lasts until the target finishes a short or long rest. 

Spellcasting. The avatar casts one of the following spells, using Intelligence as its spellcasting ability (spell save DC 16):

At will: gust, prestidigitation, thaumaturgy 

1/day each: armor of agathys, bane, banishment, blindness/deafness, darkness, mirror image, silence

Petrifying Stare (5/Day). The avatar fixes its gaze on one creature it can see within 30 feet of itself. That creature must succeed on a DC 16 Constitution saving throw or begin to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified.

Spell Reflection. If the avatar makes a successful saving throw against a spell, or a spell attack misses it, the avatar can choose another creature (including the spellcaster) it can see within 30 feet of it. The spell targets the chosen creature instead of the avatar. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.