Limited Magic Immunity. Vecna can't be affected or detected by spells of 6th level or lower unless he wishes to be. He has advantage on saving throws against all other spells and magical effects.

Unusual Nature. Vecna doesn’t need air, food, drink, or sleep.

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

Spellcasting. Vecna is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 24, +16 to hit with spell attacks). Vecna has the following wizard spells prepared:

Cantrips (at will):  Toll the Dead, Mage Hand, Mind Sliver, Prestidigitation, Shocking Grasp

1st level (6 slots): Burning Hands, Detect Magic, Magic Missle, Shield, Sleep, Thunderwave

2nd level (6 slots): Blindness/Deafness, Cloud of Daggers, Darkness, Crown of Madness, Earthbind, Hold Person 

3rd level (6 slots): Bestow Curse, Fireball, Lightning Bolt, Counterspell, Dispel Magic, Slow, Summon Undead

4th level (6 slots): Banishment, Blight, Polymorph, Stone Shape, Wall of Fire, Vitriolic Sphere

5th level (6 slots): Cone of Cold, Dominate Person, Hold Monster, Negative Energy Flood, Telekinesis, Wall of Force 

6th level (4 slot): Circle of Death, Chain Lightning, Soul Cage, Wall of Ice

7th level (3 slot): Finger of Death, Prismatic Spray, Power Word Pain

8th level (2 slot): Feeblemind, Maddening Darkness

9th level (2 slot): Power Word Kill, Meteor Swarm

Hand of Death. Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 21 (6d6) cold damage. The target must succeed on a DC 20 Constitution saving throw or be petrified for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Warp. Vecna teleports along with any equipment he is wearing or carrying, up to 60 ft to an unoccupied space he can see.

Dread Counterspell. Vecna utters a dread word to interrupt a creature he can see that is casting a spell. If the spell is 4th level or lower, it fails and has no effect. If the spell is 5th level or higher, Vecna makes an Intelligence check (DC 10 + the spell’s level). On a success, the spell fails and has no effect. Whatever the spell’s level, the caster takes 10 (3d6) psychic damage if the spell fails.

Alteration (1/day). Vecna's mastery over magic allows him to alter the very essence of another creature's spell. When he sees a damage dealing spell, he can change the damage type of that spell to a different one.

Vecna can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Vecna regains spent legendary actions at the start of his turn.

Cantrip. Vecna uses a cantrip.

Warp. Vecna uses Warp.

Spell (Costs 2 Actions). Vecna casts a spell.

Book of Vile Darkness (Costs 3 Actions). Vecna draws from the Book of Vile Darkness embedded in his chest to evoke one of the following abilities. Once an ability is used it cannot be used again until the next day.

Deathy Chill: Vecna blasts an area in a 20ft radius that he can see with a horrible cold. Creatures within the area of effect must make a DC 19 Constitution saving throw, taking 54 (8d12) cold damage and being petrified on a failed saving throw, and taking half as much damage and not being petrified on a successful saving throw.

Call of the Damned: Vecna summons 1d4 undead of CR 7 or less to aid him in battle.

Tormented Screams: Vecna calls forth the spirits of the damned to unleash their agony. Anyone that can hear their wails must make a DC 19 Intelligence saving throw, taking 36 (8d8) psychic damage and being stunned on a failed saving throw, or taking half as much damage and not being stunned on a successful saving throw.

Thaumaturgic Rejuvination: Vecna regains an expended 9th level spell slot.