Bathmanazar

Shapechanger. The imp can use its action to polymorph into a beast form that resembles a rat (speed 20 ft.), a raven (20 ft., fly 60 ft.), or a spider (20 ft., climb 20 ft.), or back into its true form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Devil's Sight. Magical darkness doesn't impede the imp's darkvision.

Magic Resistance. The imp has advantage on saving throws against spells and other magical effects.

Spellcasting. The mage is a 5th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). The mage has the following wizard spells prepared:

Cantrips (at will): control flames, create bonfire, fire bolt, minor illusion

1st level (4 slots): burning hands, charm person, shield, sleep

2nd level (3 slots): darkness, scorching ray, wither and bloom

 3rd level (2 slots): animate dead, fireball

Sting (Bite in Beast Form). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.

Invisibility. The imp magically turns invisible until it attacks or until its concentration ends (as if concentrating on a spell). Any equipment the imp wears or carries is invisible with it.

Shield. An invisible barrier of magical force appears and protects the imp. Until the start of the imp's next turn, it has a +5 bonus to AC, including against the triggering attack, and it takes no damage from magic missile.