Spellcasting. Zim is a 5th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). Zim has following Wizard spells prepared:

Cantrips (at will): Chill Touch, Mage Hand, Mending, Infestation

1st level (4 slots): False Life, Mage Armor, Witch Bolt, Shield

2nd level (3 slots): Blindness/Deafness, Misty Step, Wither and Bloom

3rd level (2 slots): Vampiric Touch,  Aura of Vitality

Dagger. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage. 

Necrotic Rend (1/Day). Zim targets one creature he can see within 80 feet and lashes out with a rend of necrotic energy. The target must succeed on a DC 14 Constitution saving throw, taking 26 (4d12) necrotic damage on a failed save or half as much on a successful one.