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

Magic Resistance. The yuan-ti has advantage on saving throws against spells and other magical effects.

Sneak Attack (1/Turn). The yuan-ti 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 yuan-ti that isn't incapacitated and the yuan-ti doesn't have disadvantage on the attack roll.

Multiattack. The yuan-ti makes two Dagger attacks and one Hand Crossbow attack, or it makes one Mind Fangs attack and then uses Soul Swap.

Dagger. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) piercing damage plus 3 (1d6) poison damage.

Hand Crossbow. Ranged Weapon Attack: +5 to hit, range 30/120 ft., one target. Hit: 6 (1d6 + 3) piercing damage, and the target must succeed on a DC 13 Constitution saving throw or be poisoned for 1 hour. If the saving throw fails by 5 or more, the target is also unconscious while poisoned in this way. The target wakes up if it takes damage or if another creature takes an action to shake it awake.

Mind Fangs. Ranged Spell Attack: +5 to hit, range 120 ft., one target. Hit: 16 (3d8 + 3) psychic damage, or instead of dealing damage the target must succeed on a DC 13 Charisma saving throw or become cursed for 1 minute. While cursed in this way, the target’s speed is reduced by 10 feet, and it must roll a d4 and subtract the number rolled from each ability check or attack roll it makes.

Soul Swap. The yuan-ti targets a creature it can see within 15 feet of it that is cursed by its Mind Fangs. Unless the target is protected by a protection from evil and good spell, it must succeed on a DC 13 Charisma saving throw or have its consciousness swapped with the yuan-ti. The yuan-ti gains control of the target’s body, and the target is unconscious for 1 hour, after which it gains control of the yuan-ti’s body. While controlling the target’s body, the yuan-ti retains its Intelligence, Wisdom, and Charisma scores and its Mind Fangs attack. It otherwise uses the controlled body’s statistics, but doesn’t gain access to the target’s knowledge, class features, or proficiencies. If the yuan-ti’s body is destroyed, both the yuan-ti and the target die. A protection from evil and good spell cast on the controlled body drives the yuan-ti out and returns the consciousness of both creatures to their original bodies.

Spellcasting. The yuan-ti casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 13):

At will: animal friendship (snakes only), minor illusion

3/day: suggestion, disguise self

2/day each: detect thoughts