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.

Burning Fangs (2/Day). The first time the yuan-ti hits with a melee attack on its turn, it can deal an extra 33 (6d10) fire damage to the target.

Eldritch Mind. The yuan-ti has advantage on Constitution saving throws that it makes to maintain its concentration on a spell.

Innate Spellcasting (Yuan-ti Form Only). The yuan-ti's innate spellcasting ability is Charisma (spell save DC 16). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only), levitate (self only)

3/day: suggestion

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

Magic Weapons. The yuan-ti’s weapon attacks are magical.

Pact Magic. The yuan-ti is a 9th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). The yuan-ti regains its spell slots after a short or long rest. It knows the following warlock spells:

Cantrips (at will): control flames, eldritch blast(300 ft. range, +4 bonus to each damage roll), message, poison spray, sacred flame, thaumaturgy

5th level (2 slots): arcane hand, create food and water, creation, enlarge/reduce, fog cloud, hellish rebuke, phantasmal killer, polymorph, protection from energy, sleep

Multiattack (Yuan-ti Form Only). The Yuan-ti makes one constrict attack and one vorpal scimitar attack.

Constrict. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage, and the target is grappled (escape DC 16) if it is a Large or smaller creature. Until this grapple ends, the target is restrained, and the yuan-ti can't constrict another target.

Vorpal Scimitar (Yuan-ti Form Only). Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 9 (1d6 + 6) slashing damage. If the target has at least one head and the yuan-ti rolled a 20 on the attack roll, the target is decapitated and dies if it can't survive without that head. A target is immune to this effect if it takes none of the damage, has legendary actions, or is Huge or larger. Such a creature takes an extra 27 (6d8) slashing damage from the hit.

Xolotl’s Vessel (4/day). The Yuan-ti targets a willing creature it can see within 100 feet and creates a tether of wispy elemental material that links the target to the yuan-ti. The tether lasts for 1 hour, until the yuan-ti uses this feature to create another tether, until the bound target is reduced to 0 hit points, or until the target ends its turn more than 100 feet from the yuan-ti. Being tethered grants the following benefits:

• The yuan-ti and the bound creature gain a +4 bonus to their Wisdom (Perception) checks.
• When the yuan-ti casts a spell, it can deliver the spell from its space or the bound creature’s space.
• The bound creature gains resistance to fire damage.