Spell Immunity. Nezerak is immune to three spells due to his magical armor. dispel magic, heat metal, and fireball

Magic Resistance. Nezerak has advantage on saving throws against spells and other magical effects.

Rejuvenation. If Nezerak is destroyed, he regains all his hit points in 1 hour unless holy water is sprinkled on his remains or a dispel magic or remove curse spell is cast on them.

Legendary Resistance (2/day). If Nezerak would fail a saving throw he can expend a use of his legendary resistance to succeed instead.

Spellcasting. The Nezerak is a 5th-level spellcaster. His spellcasting ability is charisma (spell save DC 16, +8 to hit with spell attacks). It requires no somatic or material components to cast its spells. Nezerak has the following spells prepared:

Cantrip (at will): mage hand

1st level (3 slots): magic missile, shield

2nd level (2 slots): blur, flaming sphere

3rd level (1 slot): fireball

Multiattack. Nezerak uses Fire Ray twice or makes two attacks with Nezeraks Sin and uses Soul Rend or Spectral reinforcements if able.

Fire Ray. Ranged Spell Attack: +8 to hit, range 30 ft., one target. Hit: 10 (3d6) fire damage.

Nezeraks Sin. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 15 (3d6 + 5) necrotic damage. If a creature is reduced to 0 hp by this weapon they must succeed a DC 13 constitution save or have their soul consumed by the blade.

 Soul Rend (Recharge 6). Nezerak creates a vortex of life-draining energy in a 15-foot radius centered on himself. Each humanoid in that area must make a DC 15 Constitution saving throw, taking 22 (4d10) necrotic damage on a failed save, or half as much damage on a successful one. If a humanoid creature is reduced to 0 hp this way they die and their soul is consumed by Nezerak.

Spectral Reinforcements. Nezerak summons 3 (1d4 + 1) specters in an unoccupied space within 20 ft of him. Nezerak cannot use this ability again until a humanoid creature is slain by his Soul Rend ability.

Nezerak has a pool of three legendary actions. He can use one of the following legendary actions at the end of each opponents turn, regaining all expended actions at the start of his next turn.

Weapon Attack (1 action). Nezerak makes a melee or ranged attack.

Movement (2 actions). Nezerak moves up to his speed without provoking opportunity attacks. 

Soul Syphon (3 actions). Nezerak recharges and uses his Soul Rend ability.