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. If it dies, it stays in its current form.

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

Mind Fangs (2/Day). The first time the yuan-ti hits with a melee attack on its turn, it can deal an extra 16 (3d10) psychic damage to the target.

Sseth's Blessing. When the yuan-ti reduces an enemy to 0 hit points, the yuan-ti gains 9 temporary hit points.

Innate Spellcasting (Yuan-ti Form Only). The yuan-ti's innate spellcasting ability is Charisma (spell save DC 13). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)

3/day: suggestion

Spellcasting (Yuan-ti Form Only). The yuan-ti is a 6th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 13, +5 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following warlock spells:

Cantrips (at will): eldritch blast (range 300 ft., +3 bonus to each damage roll), friends, message, minor illusion, poison spray, prestidigitation

1st–3rd level (2 slots): charm person, crown of madness, detect thoughts, expeditious retreat, fly, hypnotic pattern, illusory script

Multiattack (Yuan-ti Form Only). The yuan-ti makes one bite attack and one scimitar attack.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:5 (1d4 + 3) piercing damage plus 7 (2d6) poison damage.

Scimitar (Yuan-ti Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:6 (1d6 + 3) slashing damage.