Arcane Ninja (5e Creature)
From D&D Wiki
medium humanoid, lawful neutral
Saving Throws Wisdom +5, Dexterity +5
Assassinate. During its first turn, the arcane ninja has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the arcane ninja scores against a surprised creature is a critical hit.
Evasion. If the arcane ninja is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the arcane ninja instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Sneak Attack (1/Turn). The arcane ninja deals an extra 7 (2d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the arcane ninja that isn't incapacitated and the arcane ninja doesn't have disadvantage on the attack roll.
Magical Weapons. All of the arcane ninja's weapon attacks are considered to be magical.
Spellcasting. The arcane ninja is an 8th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). The arcane ninja has the following spells prepared:
Shortsword. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much on a successful one.
Arcane ninjas are a variant on the assassin in the Monsters Manual of the official content with spellcasting added. Its not a complex modification, but it is useful for certain situations.