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.

Innate Spellcasting (Yuan-ti Form Only). The yuan-ti’s innate spellcasting ability is Charisma (spell save DC 14). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)

3/day: suggestion

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

Two Heads. The Yuan-ti has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

CorrodingStrike (2/Day). The first time the yuan-ti hits with a melee attack on its turn, it can deal an extra  22(4d10) acid damage to the target.

Spellcasting (Yuan-ti Form Only). The yuan-ti is an 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +6 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, minor illusion, mage hand, poison spray, prestidigitation

1st–3rd level (2 4th-level slots): disguise self(at will), guiding bolt, charm person, hex, scorching ray, mirror image, fear, counterspell, wall of fire, hallucinatory terrain

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

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) piercing damage plus 7 (2d6)poison damage.

Claw (Yuan-ti Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage

Scimitar (Yuan-ti Form Only). Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) slashing damage.