Aspect of Vecna

Deific Resistance. Vecna automatically succeeds on saving throws against spells and other magical effects, and the attack rolls of spells automatically miss him.

God of Magic and Secrets. Vecna has Advantage on saving throws to maintain Concentration, his spells and other magical effects ignore Resistance to Necrotic damage, and he knows when he hears a lie and senses whenever a creature learns or reveals any secret that affects the lives of at least 500 creatures. If a creature attempts to cast a spell that would banish, imprison, mind-control, or shape-shift Vecna; detect or read Vecna's emotions, thoughts, or location, it suffers the following effect. Charisma Saving Throw: DC 27. Failure: Vecna learns the target's greatest fear, desire, and secret. Failure or Success: 14 (4d6) Psychic damage.

Legendary Resistance (5/Day). If Venca fails a saving throw, he can choose to succeed instead.

Preparedness. After taking a Short or Long Rest, Vecna can replace any level 1+ spell in his Spellcasting action with another spell of the same level from the Cleric or Wizard spell list.

Special Equipment. Vecna weilds the following items:

Afterthought. Vecna carries a magic dagger named Afterthought. In the hands of anyone other than Vecna, Afterthought is a +3 Dagger.
Book of Vile Darkness. While attuned to the Book of Vile Darkness, Vecna has Advantage on Intelligence checks.
Eye and Hand of Vecna. While attuned to the Eye and Hand of Vecna, Vecna has Advantage on Initiative rolls and regains 20 Hit Points at the start of each of its turns if he starts his turn with at least 1 Hit Point. Additionally, Vecna has X-ray vision with a range of 60 feet. To Vecna, solid objects within that radius appear transparent and don’t prevent light from passing through them. The vision can penetrate 1 foot of stone, 1 inch of common metal, or up to 3 feet of wood or dirt. Thicker substances block the vision, as does a thin sheet of lead.

Undying King. If Vecna is destroyed, his body disintegrates, and he is unable to create a new aspect for 1d100 years. Vecna’s new aspect appears in an unoccupied space in his lair with all his Hit Points.

Multiattack. Vecna uses Necrotic Reduction and makes four Afterthought attacks. He can replace three attacks with a use of (A) Flight of the Damned if available, (B) Rotten Fate, or (C) Spellcasting to cast Dispel Magic.

Afterthought. Melee Attack Roll: +18, reach 5 ft. Hit: 14 (2d4 + 9) Piercing damage, plus 9 (2d8) Necrotic damage and 9 (2d8) Cold damage. If the target is a creature, it suffers the following effect. Constitution Saving Throw: DC 27. Failure: The target can't regain Hit Points, and the target repeats the save at the end of each of its turns, ending the effect on itself on a success.

Necrotic Reduction. Constitution Saving Throw: DC 27, one creature Vecna can see within 5 feet. Failure: 56 (16d6) Necrotic damage. Success: Half damage. Failure or Success: A creature reduced to 0 Hit Points by this damage melts into a slime the same size as the creature. Whenever a creature enters the slime's space or ends its turn there, it takes 11 (2d10) Acid damage.

Rotten Fate. Constitution Saving Throw: DC 27, one creature Vecna can see within 120 feet. Failure: 123 (14d8 + 60) Necrotic damage. Success: Half damage. Failure or Success: A Humanoid killed by this damage rises as a Zombie. The zombie is under Vecna's control and shares his Initiative count, but acts immediately after Vecna.

Flight of the Damned (Recharge 5-6). Constitution Saving Throw: DC 27, each creature in a 120-foot Cone. Failure: 45 (10d8) Necrotic damage. If the target is a creature, it has the Frightened condition for 1 minute, and the target repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically. Success: Half damage only.

Unmaking Wish (1/30 Days). Charisma Saving Throw: DC 27, Vecna while casting Wish within 30 feet of the Sword of Kas and attuned to the Eye and Hand of Vecna. Failure: Nothing happens, and the Wish spell is wasted. Success: The Sword of Kas is destroyed.

Spellcasting. Vecna casts one of the following spells, requiring no Material components, and using Intelligence as the spellcasting ability (spell save DC 27).

At Will: Animate Dead (level 9 version, as an action), Clairvoyance, Crown of Madness, Detect Magic, Detect Thoughts, Dimension Door, Dispel Magic, Fly, Invisibility, Mage Hand, Prestidigitation, Sleep, Slow
3/Day Each: Chain Lightning, Circle of Death, Disintegrate, Eyebite, Fireball (level 5 version), Globe of Invulnerability, Lightning Bolt (level 5 version), Scrying (as an action)
2/Day Each: Dominate Monster, Finger of Death, Plane Shift, Power Word Kill, Teleport

Dread Teleport. Vecna teleports up to 30 feet to an unoccupied space he can see. Each creature of Vecna's choice within 20 feet of his destination space takes 14 (4d6) Psychic damage. If at least two creatures take this damage, Vecna regains 120 hit points.

Vecna can take up to three Reactions per round.

Fell Rebuke. Trigger: Vecna takes damage from a creature within 150 feet. Response: The triggering creature takes 14 (4d6) Necrotic damage.

Reactive Magic. Vecna casts Counterspell, Shield, Silvery Barbs, or Soul Cage in response to the spell’s trigger, using the same spellcasting ability as Spellcasting.

Legendary Action Uses: 4 (5 in Lair). Immediately after another creature’s turn, Vecna can expend a use to take one of the following actions. Vecna regains all expended uses at the start of each of his turns.

Antimagic Chill. Intelligence Saving Throw: DC 27, one creature Vecna can see within 60 feet. Failure: 21 (6d6) Cold damage plus 21 (6d6) Force damage. Until the end of the target's next turn, it gains 1 level of Exhaustion, can't take the Magic action, and its Speed is halved. Success: Half damage only. Failure or Success: Vecna can't take this action again until the start of his next turn.

Escape. Vecna uses Spellcasting to cast Fly, Dimension Door, or Invisibility. Vecna can’t take this action again until the start of his next turn.

Stupefy. Vecna uses Spellcasting to cast Crown of Madness, Sleep, or Slow. Vecna can't take this action again until the start of his next turn.

Vile Speech. Each creature that isn't a Fiend or an Undead in a 15-foot Enemation originating from Vecna while he's attuned to the Book of Vile Darkness takes 14 (4d6) Psychic damage.