God of disease. Nurgul's weapon attacks deal an extra 6d6 necrotic damage (included in the attack).

Shapeshift. Nurgul shapeshifts into a Medium Humanoid or into a Swarm of Insects, or back into his true form. His statistics stay the same in each form except his size and monster type.

Unstoppable force. Difficult Terrain doesn't slow Nurgul's speed and Dangerous Terrain deals half damage to him.

Innate Spellcasting. Nurgul can innately cast the following spells. He uses Wisdom as his spellcasting modifier and doesn't require any material components:
At will: Bestow Curse, Blight, Chill Touch, Raise Dead
3/day each: Circle of Death, Contagion
1/day: Abi-Dalzim's Horrid Wilting

Divine nature. Nurgul does not require food, water or sleep.

Multiattack. Nurgul makes 4 melee attacks. He can use his Innate Spellcasting in place of two of the attacks.

Plaguereaper. Melee Weapon Attack: +18 to hit, reach 10ft., 1 target. Hit: 87 (12d10 + 15) Slashing plus 24 (6d6) Necrotic damage. 

Nurgul can take up to 3 legendary actions at the end of another creature's turn. He regains spent legendary actions at the start of his turn

Infest (Costs 1 Action). Nurgul casts Infestation without any material, somatic or verbal components required.

Move (Costs 1 Action). Nurgul moves up to half his speed.

Pestilent Cloud (Costs 2 Actions). Nurgul releases a cloud of fetid smoke in a 60ft cone originating from himself. All creatures inside the cone must make a DC 24 CON save or take 40 (8d8) poison damage.

Nurgul can take 1 mythic action at the start of his turn if he has under half of his max HP remaining. He regains spent legendary actions after a long rest.

Pestilence Embodied (Costs 1 Action). Nurgul speaks a foul incantation. Until the start of his next turn, he halves all damage dealt against him, has advantage on all ability checks, is resistant to all damage, and the Pestilent Cloud Legendary action only costs one action.