Devil's Sight. Magical darkness doesn't impede the abishai's darkvision.
Magic Resistance. The abishai has advantage on saving throws against spells and other magical effects.
Multiattack. The abishai makes two Fiendish Claw attacks, or it makes one Fiendish Claw attack and uses Spellcasting.
Fiendish Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) force damage. If the target is a creature, it must succeed on a DC 16 Constitution saving throw or take 16 (3d10) poison damage and become poisoned for 1 minute. The poisoned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Spellcasting. The abishai casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 17):
At will: alter self, major image