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.

Duel Minds. The yuan ti an concentrate on two spells at once. 

Innate Spellcasting (Duel Mind Form Only). The yuan-ti’s innate 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: suggestion

1/day: fear

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

Spellcasting. The yuan ti is a 10th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). The yuan ti has the following wizard spells prepared:

Cantrips (at will): mage hand, minor illusion, poison spray, ray of frost

1st level (4 slots): mage armor, magic missile, shield, witch bolt

2nd level (3 slots): hold person, misty step, flaming sphere

3rd level (3 slots): enemies abound, lightning bolt

4th level (3 slots): Evard’s black tentacles, greater invisibility

5th level (2 slots): dominate person

Dagger. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 2 (1d6 + 2) Piercing damage plus 3 (1d6) poison damage.