Devil's Sight. Magical darkness doesn't impede Zuelk's darkvision.

Spellcasting. Zuelk is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). Zuelk has following Wizard spells prepared:
Cantrips (at will): fire bolt, ray of frost, toll the dead
1st level (4 slots): magic missile, shield, witch bolt, thunderwave 
2nd level (4 slots): darkness, hold person, misty step, shatter
3rd level (3 slots): counterspell, bestow curse, lightning bolt
4th level (3 slots): banishment, wall of fire
5th level (3 slots): dominate person, enervation
6th level (2 slots): chain lightning, mental prison
7th level (2 slots): finger of death, forcecage, power word pain
8th level (2 slots): maddening darkness, feeblemind, antimagic field
9th level (1 slots): psychic scream

Magical Weapon. Zuelk's staff is considered magical and is used as a focus for his spellcastings.

Demon's Blast. Ranged Spell Attack: +12 to hit, range 60ft., one target. Hit: 19 (3d8 + 5) necrotic damage.

Staff of the High Mage. Melee weapon attack: +8 to hit, range 5ft., one target. Hit: 16 (3d6 + 5) magical bludgeoning damage.