Celestial

Bane of the nonbeliever: those who are the enemy of its master lose 5 health as they get closer 

Spellcasting. Celestial is a level 10 wizard . Its spellcasting ability is wisdom (spell save DC 18, + 18 to hit with spell attacks). Celestial has following wizard spells prepared:
Cantrips (at will): acid splash, fire bolt, frost bite
1st-4th level (15 slots): darkness, lightning bolt, remove curse, dispel magic, magic weapon, sunlight

Ring of pain: when within 20ft of celestial everyone but the master of celestial must make a Wis save of 18 or the lose half their health 

voltaic pulse: the target gains 4d10 health 

After every 2nd cantrip it casts 

caller of carnage: instantly cast sunlight but it has half the amount of dice