Spellcasting. Zaroff Umbri is a 9-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). Zaroff has following warlock spells prepared:
Cantrips (at will): [Green Flame Blade], [Eldritch Blast], [Blade Ward] 
5th level (2 slots): [Armor of Agathys], [Expeditious Retreat], [Shadow Blade], [Fear], [Vampiric Touch], [Summon Lesser Demons], [Shadows of Moil], [Danse Macabre], [Negative Energy Flood], [Misty Step] 

Voice of the Void. As a action Zaroff may invoke the name of the Void to call up up six Verminus or one Cogak'Y. Once Zaroff uses this he must wait 1 day before invoking the void again. 

Multiattack. Zaroff makes two attacks with his repeater pistols or two attacks with his chainsword.

Chainsword. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 3 (2d4 - 1 ) [slashing] damage. 

Rotor Pistol. Ranged Weapon Attack: +7 to hit, range 25/40 ft., 1 target. Hit: 8 (1d8 + 4) [piercing] damage or Hit: 13 (3d6 + 4) [piercing] damage Roll a 1d8 on a 1 the Rotor Pistol explodes dealing 2d6 Fire damage to the wielder.