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

Acidic Blood. When the Disciple takes piercing or slashing damage, each creature within 5 ft. of the Disciple takes 4 (1d6) acid damage.

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

Innate Spellcasting (Abomination Form Only). The yuan-ti’s innate spellcasting ability is Charisma (spell save DC 15). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only), acid splash

3/day: suggestion, ensnaring strike

2/day: acid arrow

1/day: fear

Multiattack (Abomination Form Only). The yuan-ti makes two ranged attacks or three melee attacks, but can use its bite attack only once.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) poison damage.

Scimitar (Abomination Form Only). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Longbow (Abomination Form Only). Ranged Weapon Attack: +6 to hit, range 150/600 ft., one target. Hit: 12 (2d8 + 3) piercing damage plus 10 (3d6) poison damage.

Villain Actions

On initiative 20 while the Disciple is alive, it takes a villain action depending on how many rounds it has been in the initiative.

Round One

Yuan-ti Escape. The Disciple along with any equipment he is wearing or carrying and any creature they are currently constricting, teleports to an unoccupied space within 30 ft. that he can see. Everything within 10 ft. radius of where the Disciple teleported from is splashed with acid. Each creature in the area must make a DC 14 Dexterity saving throw, taking 10 (2d8 + 3) acid damage on failed save, or half as much damage on a successful one. 

Round Two

Glob Splash. The Disciple hurls acid globs at three targets. Ranged Weapon Attack: +6 to hit, range 30/60 ft., one target. Hit: 10 (2d8 + 3) acid damage. Target must succeed on a DC 14 Strength saving throw or be grappled until its next turn.

Round Three

Acid Thrash. The Disciple shakes their body spraying acid blood from their wounds in all directions and splashing everything within 30 ft., each creature in the area must make a DC 14 Dexterity saving throw, taking 10 (2d8 + 3) acid damage on failed save, or half as much damage on a successful one. 

Ensnaring Strike. Cast ensnaring strike

Constrict. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage, and the target is grappled (escape DC 14). Until this grapple ends, the target is restrained, and the yuan-ti can’t constrict another target.

Acid Glob. If an enemy leaves the Disciples reach it can hurl a glob of acid. Ranged Weapon Attack: +6 to hit, range 30/60 ft., one target. Hit: 10 (2d8 + 3) acid damage. Target must succeed on a DC 14 Strength saving throw or be grappled until its next turn.

Death Throes. As it dies the Disciple sprays everything within 30 ft. with acid blood. Each creature in the area must make a DC 14 Dexterity saving throw, taking 10 (2d8 + 3) acid damage on failed save, or half as much damage on a successful one.