Cunning Action. On each of its turns, the spy can use a bonus action to take the Dash, Disengage, or Hide action.

Innate Spellcasting. 

The yuan-ti's spellcasting ability is Charisma (spell save DC 15). The yuan-ti can innately cast the following spells, requiring no material components: At will: animal friendship(Snakes only)

3/day each: poison spray, suggestion

Sneak Attack(1/turn). The spy deals an extra 14(4d6) when it hits a target with a weapon attack and the spy has advantage on the attack roll, or when the target is within 5 ft of an ally of the spy that is not incapacitated and the spy doesn't have disadvantage on the attack roll.

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

Murder. During its first turn in a combat the spy has advantage on attack rolls against a creature that hasn't taken a turn yet.

Multiattack. The spy makes two melee attacks.

Shortsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage plus 10(3d6) poison damage.

Shortbow. Ranged Weapon Attack: +6 to hit, range 80/320 ft., one target. Hit: 6 (1d6 + 3) piercing damage plus 10(3d6) poison damage.