Shapechanger: The Yuan-ti can use its action to polymorph into a large Venemous Snake or back to its true form. Its stats are the same in either form. If it hits 0hp it stays in it's current form.

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

Mind Fangs (3/day): the first time the Mind Shrieker hits with a melee attack on its turn, it can deal an extra 16(3d10) psychic damage to its target.

Innate Spellcasting: The yuan-ti's innate spellcasting dc is 16. Its spellcasting ability is Charisma. It can innately cast the following spells.

At will: Command, Minor Illusion, Animal Friendship(snakes only)

3/day: Dissonant Whispers, Suggestion

1/day: Mass Suggestion

Spellcasting. The Mind Shrieker is a 9th-level spellcaster. Its spellcasting ability is Charisma(spell save DC 16, +8 to hit with spell attacks). The Mind Shrieker has following spells prepared:
Cantrips (at will): Eldritch Blast[+4 to damage, 300 ft], Poison Spray, Mind Blast*, Psychic Blades*, Toll the Dead
1st level (4 slots): Chromatic Orb, Dissonant Whispers, Tasha's Hideous Laughter, Bane.

2nd level (3 slots): Crown Of Madness, Enhance Ability, Silence.

3rd Level (3 slots): Crush Mind(6d6)*, Hypnotic Pattern, Vampiric Touch.

4th Level (3 slots): Dimension Door, Confusion, Compulsion, Agony*, Vitriolic Sphere

5th Level (2 Slots): Seeming, Dominate Person.

*homebrew spell. Available on this site.

Sseth's Blessing: When this creature reduces an enemy to 0 hp, it gains 10 temporary hit points.

Multiattack: The Mind Shrieker attacks twice with its Scimitars and once with its Bite

Scimitar. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 8 (1d6 + 4) Slashing damage. (Plus an additional 1d6 psy if psychic blades is active)

Bite. Melee Attack: +7 to hit, reach 5 ft., 1 target. Hit: 7 (2d4 +4) Piercing Damage plus 7 (2d6) Poison damage.