Avatar of Velsharoon

Legendary Resistance (3/Day). If the Avatar of Velsharoon fails a saving throw, it can choose to succeed instead.

Unattached. The avatar is an illusion, instead, is being powered by two giant gems inside of its eyes, each with its own armor class and hit die. The only way to discern this illusion is by actually bypassing the armor class of the Avatar.
    the skull shimmers as your attack does nothing, phasing  through it. The gemstones remain prominent, staring into your soul.

Ruby encrusted diamond(15,000gp); 130 hit points, 18 AC, immune to nonmagical attacks.
Resistant against Fire, Thunder, Poison, Psychic, and Force damage.
Vulnerable against Radiant
Emerald encrusted opal(20,000gp); 164 hit points, 17 AC, immune to nonmagical attacks.
Resistant against Cold, Necrotic, Acid, Psychic, and Force damage.
Vulnerable against Radiant

The diamond and opal may hold separate concentrations for spells. If the avatar takes damage from any source, and would lose concentration due to failing a concentration check,  the diamond gemstone loses it's concentration (if any) before the opal would.

Spellcasting. The Avatar of Velsharoon is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The avatar has the following wizard spells prepared:

Cantrips (at will): minor illusion, toll the dead, ray of frost

1st level (4 slots): absorb elements, magic missile, shield, sleep

2nd level (3 slots): blindness/deafness, detect thoughts, invisibility, alter self

3rd level (3 slots): animate dead, counterspell, dispel magic, fireball

4th level (3 slots): blight, dimension door

5th level (3 slots): wall of force, cone of cold

6th level (1 slot): contingency, create undead

7th level (1 slot): finger of death, etherealness

8th level (1 slot): feeblemind, antimagic field

9th level (1 slot): invulnerability

Turn Resistance. The Avatar of Velsharoon has advantage on saving throws against any effect that turns undead.

Paralyzing Aura. The Avatar of Velsharoon emits an aura of evil, chilling even the bravest to their bones. Each creature of the avatar's choice within 120 feet and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute upon entering its aura. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on a success. If a creature's saving throw is successful or if the effect ends for it, the creature is immune to the Avatar’s Paralyzing Aura for the next 24 hours.

Undead Presence: The Avatar of Velsharoon may let out a wave of undeath, raising any corpses within 120 ft as skeletons or zombies, with the original health points of the creature that died.

Necrotic Blast. Ranged Attack. +12 to hit, range 60 ft., one creature. Hit: 62 (10d10) +7 necrotic damage. An orb of necrotic energy is lobbed at the target. any creatures within 5 ft. of the target must succeed a DC. 18 Dexterity saving throw or take half the original damage.

Death Blight. Aura. Range 60 ft. radius around the avatar. An area around the avatar becomes vile as a necrotic energy rises up from the ground. All creatures that aren't Undead, must succeed a DC. 18 constitution saving throw or take 38 (7d8) +7 necrotic damage, half if succeeding. Any creatures who fail the save receive half healing until the avatar's next turn.

Mastered Arts. When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

The lich can take 3 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. The lich regains spent legendary actions at the start of its turn.

Cantrip. The avatar casts a cantrip.

Disengage. The avatar may disengage and move up to its move speed without provoking an opportunity attack

Undead Gaze (Costs 2 Actions). The avatar's opal eye creates a necrotic zone in a 120-foot cone. this effect lasts until the start of the avatar's next turn. Each creature must make a DC Dexterity or take 10d10(55) necrotic damage, halved if successful. If the Opal has been destroyed, the avatar may not use this ability.

Antimagic Gaze (Costs 2 Actions). The avatar's diamond eye creates an area of antimagic, as in the anti magic field spell, in a 120-foot cone. This effect lasts until the start of the avatar's next turn. If the diamond has been destroyed, the avatar may not use this ability.

Disrupt Life (Costs 3 Actions). Each non-undead creature within 30 feet of the avatar must make a DC 16 Constitution saving throw against this magic, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.