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

Shapechanger. The Yuan-ti can use its action to polymorph into a Huge 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 (Emperor form only). The Yuan-ti's innate spellcasting ability is Charisma (spell save DC 20). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)
3/day: suggestion
2/day: fear
1/day: poison spray

Multiattack (Emperor form only). The  makes five attacks: four with its scimitars and one with its tail or one with its bite

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 10 (2d6+4) piercing damage plus 14 (4d6) poison damage

Scimitar (Emperor form only). Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 4) slashing damage.

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., one creature. Hit: 15 (2d10 + 4) bludgeoning damage. If the target is Medium or smaller, it is grappled (escape DC 19). Until this grapple ends, the target is restrained, the Yuan-ti can automatically hit the target with its tail, and the marilith can't make tail attacks against other targets.

Teleport (recharge 5-6). The Yuan-ti magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

Parry. The yuan-ti adds 5 to its AC against one melee attack that would hit it. To do so, the yuan-ti must see the attacker and be wielding a melee weapon.