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

Shadowcoil Form (1/day) The Yuan-Ti Night Abomination assumes a shadowy coiled form. For 1 minute the Night Abomination gains the following: 

Resistance to all Damage except Radiant.

Can move through creatures as if they are difficult terrain, leaving behind a 10-ft. trail of nightmare ichor. Anyone starting their turn in it must succeed a DC 14 wisdom saving throw or be frightened for 1 turn. The Ichor disappears after 1 minute. 

Multiattack. The yuan-ti makes two Bite attacks, and it can use Spellcasting to cast Suggestion if available.

Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 11 (2d6 + 4) Piercing damage plus 10 (3d6) Poison damage.

Constrict. Strength Saving Throw:  DC 15, one Large or smaller creature within 5 feet. Failure: 28 (7d6 + 4) Bludgeoning damage. The target has the Grappled condition (escape DC 14), and it has the Restrained condition until the grapple ends. Success: Half damage only.

Poison Spray (Recharge 5–6). Constitution Saving Throw: DC 14, each creature in a 30-foot Cone. Failure: 21 (6d6) Poison damage, and the target has the Poisoned condition until the end of the yuan-ti’s next turn. While Poisoned, the target has the Blinded condition. Success: Half damage only.

Spellcasting (Yuan-ti Form Only). The yuan-ti casts one of the following spells, requiring no Material components and using Wisdom as the spellcasting ability (spell save DC 15):

At Will: Animal Friendship (snakes only)
3/Day: Suggestion

Shape-Shift. The yuan-ti shape-shifts into a Large snake or returns to its true form. If it dies, it stays in its current form. The yuan-ti’s game statistics are the same in each form, except where noted. Any equipment it is wearing or carrying isn’t transformed.

Reactive Sting. If the Yuan-Ti Night Abomination is hit with a melee attack from a creature within 5ft., it can use its reaction to make a Poisonous Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 11 (2d6 + 4) Piercing damage plus 10 (3d6) Poison damage.