Shapechanger. The yuan-ti can use its action to polymorph into a great winged 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.

Devil's Sight.  Magical darkness does not impede the yuan-ti hag's vision.

Blind Sight.  The hag is aware of any invisible creature that steps within 30' of her.

Ophidiophobia Aura. Any creature of the anathema’s choice, other than a snake or a yuan-ti, that starts its turn within 30 feet of the anathema and can see or hear it must succeed on a DC 17 Wisdom saving throw or become frightened of snakes and yuan-ti. A frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target’s saving throw is successful or the effect ends for it, the target is immune to this aura for the next 24 hours.

Magic Resistance. The yuan-ti hag has advantage on saving throws against spells and other magical effects that are directed at her.

Innate Spellcasting. The yuan-ti hag’s innate spellcasting ability is Charisma (spell save DC 15) or +9 to hit. The hag can innately cast the following spells, requiring no material components:

• At will: animal friendship (snakes only), chill touch, 
• 3/day: suggestion
• 1/day: fear

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

Enspelled Staff.  The yuan-ti hag is in possession of an enspelled staff.  It is a very rare version, and is enspelled with Shadow of Moil.  She can cast this spell multiple times, as an action.

Multiattack. The yuan-ti hag makes two melee attacks (staff), or can cast Burning Touch or Flaming Orb.

Staff.  Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 10 (2d6 + 4) bludgeoning damage plus 10 (3d6) necrotic damage.

Burning Touch. Melee Spell Attack: +9 to hit, reach 5 ft., one target. Hit: 14 (3d6 + 4) fire damage.

Flaming Orb. Ranged Spell Attack: +9 to hit, range 90 ft, one target. Hit: 25 (5d6 + 4) fire damage.

The yuan-ti hag can take 3 legendary actions, choosing from the options below.  Only one legendary action option can be used at a time, and only at the end of another creature's turn.  The hag regains spent legendary actions at the beginning of her turn.

Hellish Chains (Costs 3 Actions). Severin targets one creature he can see within 30 feet of him. The target is wrapped in magical chains of fire and restrained. The restrained target takes 10 (3d6) fire damage at the start of each of its turns. At the end of its turns, the target can make a DC 17 Strength saving throw, ending the effect on itself on a success.

Attack.  The hag makes one attack.

Disengage.  The hag can disengage and move up to half her movement.

Fiery Teleport (Costs 2 Actions).  The hag, along with any objects she is wearing or carrying, teleports up to 40 feet to an unoccupied space she can see.  Each creature within 5 feet of her before she teleports takes 5 (1d10) fire damage.