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.

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

Spellcasting. The Yuan-ti Tempest Priest is a 7-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 14, +5 to hit with spell attacks). The Yuan-ti Tempest Priest has following spells prepared: 
Cantrips (at will):  Word of Radiance and Thunderclap
1st level (4 slots): Thunderwave and Guiding Bolt 2nd level (3 slots) Prayer of healing and Silence 3rd level (2 slots) Gust of Wind, Shatter 4th level (1 slot) Banishment

Multiattack (Yuan-ti Form Only). The yuan-ti makes two melee attacks

Action Melee Attack. Mace. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) slashing damage.

Action Ranged Attack. Light Crossbow. Ranged Weapon Attack: +4 to hit, range 80/320 ft., one target. Hit: 4 (1d8 + 1) piercing damage

Wrath of the Storm. You can thunderously rebuke attackers. When a creature within 5 feet of you that you can see hits you with an attack, you can use your reaction to cause the creature to make a Dexterity saving throw. The creature takes 2d8 lightning or thunder damage (your choice) on a failed saving throw, and half as much damage on a successful one.

You can use this feature a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest.