Devil's Rune. Zudal possesses a blood rune inscribed on the staff he carries around with him, though to any other smaller creature - it is a great club. While holding or wearing the object with the rune, he may use Corrupting Hex as an action and Poisoning Rebuke as a reaction. 

The staff bearing the blood rune has an AC 22, 30 hit points; immunity to psychic, necrotic, radiant, and poison damage. The object regains all its hit points at the end of Zudal's turn, but turns to dust and is destroyed if Zudal dies. 

Devil's Sight. Magical darkness does not impede Zudal's Darkvision. 

Legendary Resistance (3/Day). If Zudal fails a saving throw, he may choose to succeed instead. Spent legendary resistances are regained after a long rest. 

Spellcasting. Zudal's spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks). Zudal has the following spells prepared: 
(At will): Detect Magic, Disguise Self, Enlarge/Reduce, Mage hand, Mage Armor. 
(1 / Day Each): Chain Lightning, Cone of Cold (6th-level version), Detect Thoughts, Erupting Earth (6th-level version), Fireball (6th-level version). 

Multiattack. Zudal makes three Staff attacks, and if he still has the Blood Rune active, he may replace one of these actions with Corrupting Hex. 

Devil Staff. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: (3d6 + 6) bludgeoning damage plus an additional (2d10) force damage. 

Corrupting Hex. Zudal targets one creature he can see within 60 feet of himself. The target must succeed on a DC 17 Charisma saving throw or take (6d8) necrotic damage and become cursed for 24 hours. While cursed this way, the target's speed is reduced by half, and if it tries to cast a spell, it must first succeed on a DC 18 Intelligence check or the spell fails and is wasted. 

Eldritch Burst. Magical energy explodes in a 20-foot-radius sphere centered on a point Zudal can see within 120 feet of himself. Each creature in that area must succeed on a DC 18 Dexterity saving throw. On a failed save, the creature takes (4d12) force damage and has the prone. On a successful save, it takes half damage only. 

Creeping Gloam (Recharge 5-6). Zudal temporarily conjures a grasping darkness in a 30-foot-radius sphere centered on a point he can see within 120 feet of himself. Each creature in that area must succeed on a DC 18 Constitution saving throw or make (2d10) necrotic damage and have the blinded condition until the end of its next turn.

Poisoning Rebuke (requires Devil's Staff). As a result of taking damage from a target Zudal can see within 60 feet of himself, Zudal can force that creature to succeed on a DC 18 Constitution saving throw or take (2d10) necrotic damage and have the Blinded condition until the end of its next turn.