Shapechanger. The yuan-ti can use its action to polymorph into a Medium snake, or back into its true form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It doesn't change form if it dies.

Innate Spellcasting (Yuan-ti Form Only). The yuan-ti's innate spellcasting ability is Charisma (spell save DC 13). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)
3/day: suggestion

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

Multiattack. The Malison makes two ranged attacks or two melee attacks, but can constrict only once.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 6) piercing damage.

Constrict. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 6) bludgeoning damage, and the target is grappled (escape contested Strength Check). Until this grapple ends, the target is restrained, and the yuan-ti cannot constrict another target.

Halberd. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 7 (1d10 + 6) slashing damage.

Longbow. Ranged Weapon Attack: +8 to hit, reach 150/600 ft., one target. Hit: 6 (1d8 + 6) piercing damage.

Venomous Coating. As a Bonus Action, the Yuan-Ti Malison can secrete lingering venom from it's fangs to coat a weapon. On hit, the target must succeed on a Constitution Saving throw (DC 15) or become poisoned form 1 minute. At the start of each of their turns, a poisoned target takes 1d8 poison damage. At the end of each of their turns, a poisoned target can roll their Constitution saving throw again. On a success, the target is no longer poisoned.