Medium humanoid (any race), any non-good alignment
Saving Throws Dex +6, Int +4
Skills Acrobatics +6, Deception +3, Perception +3, Stealth +9
Damage Resistances poison
Senses passive Perception 13
Languages Thieves' cant plus any two languages
Challenge 8 (3,900 XP) Proficiency Bonus +3
Traits
Assassinate. During its first turn, the assassin has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.
Evasion. If the assassin is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the assassin instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Sneak Attack (1/Turn). The assassin deals an extra 13 (4d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 ft. of an ally of the assassin that isn't incapacitated and the assassin doesn't have disadvantage on the attack roll.
Actions
Multiattack. The assassin makes two shortsword attacks.
Shortsword. Melee Weapon Attack: +6 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 damage on a successful one.
Light Crossbow. Ranged Weapon Attack: +6 to hit, range 80/320 ft., one target. Hit: 7 (1d8 + 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 damage on a successful one.