Shapechanger. The yuan-ti can use its action to polymorph into a large 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. The yuan-ti's innate spellcasting ability is Wisdom (spell save DC 19). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)

3/day: suggestion

1/day: fear

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

Agile Parry. If the yuan-ti makes an unarmed strike as part of the Attack action on its turn and is holding a weapon, it can use it to defend itself if it is a melee weapon. It gains a +2 bonus to AC until the start of its next turn, while the weapon is in its hand, and it isn't incapacitated.

Unerring Accuracy. The yuan-ti's mastery of weapons grants it extraordinary accuracy. If it misses with an attack roll using a weapon on its turn, it can reroll it. It can use this feature only once on each of its turns. 

Ki-Empowered Strikes. The yuan-ti's unarmed strikes count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Evasion. The yuan-ti's instinctive agility lets it dodge out of the way of certain area effects. When it's subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Legendary Resistance (3/Day). If the the yuan-ti fails a saving throw, it can choose to succeed instead.

Stillness of Mind and Body. The yuan-ti can use its action to end one effect on itself.

Multiattack. The yuan-ti makes three attacks but can use its bite and constrict attacks only once each.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) poison damage.

Constrict. Melee Weapon Attack: +10 to hit, reach 10ft., one target Hit: 11 (2d6 + 4) bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the yuan-ti can't constrict another target.

Nodachi. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage. 

Unarmed Strike. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 14 (1d10 + 8) bludgeoning damage plus 6 (1d10) fire damage.

Blade Throw. Ranged Weapon Attack: +14 to hit, range 50/100 ft., one target. Hit: 12 (2d6 + 8) piercing damage. Starting at the beginning of the yuan-ti's turn the sword begins to hover, flies up to 30 feet, and attacks one creature of the yuan-ti's choice within 5 feet of it. The sword uses the yuan-ti's attack roll and ability score modifier to damage rolls. After six rounds the sword returns to its scabbard.

Flurry of the Rising Sun. Immediately after the yuan-ti takes the Attack action on its turn, it can make two unarmed strikes as a bonus action. Recharge (5-6).

Deflect Missiles. The yuan-ti can use its reaction to deflect or catch the missile when it is hit by a ranged weapon attack. When it does so, the damage it takes from the attack is reduced by 1d10+8+20. If it reduces the damage to 0, it can catch the missile if it is small enough for it to hold in one hand and it has at least one hand free. If it catches a missile in this way, it makes a ranged attack with the weapon or piece of ammunition it just caught, as part of the same reaction. It makes this attack with proficiency, and the missile counts as an unarmed weapon for the attack.

The yuan-ti can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The yuan-ti regains spent legendary actions at the start of its turn.

Unarmed Strike. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 14 (1d10 + 8) bludgeoning damage plus 6 (1d10) fire damage.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) poison damage.

Constrict (Costs 2 Actions). Melee Weapon Attack: +10 to hit, reach 10ft., one target Hit: 11 (2d6 + 4) bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the yuan-ti can't constrict another target.

Blade Throw. (Costs 3 Actions) Ranged Weapon Attack: +14 to hit, range 50/100 ft., one target. Hit: 12 (2d6 + 8) piercing damage. Starting at the beginning of the yuan-ti's turn the sword begins to hover, flies up to 30 feet, and attacks one creature of the yuan-ti's choice within 5 feet of it. The sword uses the yuan-ti's attack roll and ability score modifier to damage rolls. After six rounds the sword returns to its scabbard.

Call of the Sun God (1/Day). The yuan-ti calls out to its God and a fiery explosion emanates from its position. Each creature in a 100-foot-radius sphere centered on that point must make a Dexterity saving throw (DC 16). A target takes 20d6 fire damage on a failed save, or half as much damage on a successful one.

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.