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.

Innate Spellcasting (Abomination Form Only). The yuan-ti's innate spellcasting ability is Charisma (spell save DC 13). The yuan-ti can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)
2/day each: suggestion, command (Grovel only)
1/day each: fear, wrathful smite

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

Frenzied Action (1/day). On its turn, the yuan-ti can take an additional action. Immediately after it does so, it gains 3 levels of exhaustion.

Brute. A melee weapon deals an one extra die of damage when the yuan-ti hits with it (included in the attack)

Multiattack (Enforcer form only). the yuan-ti can make two melee attacks with its halberd and one with its constrict. It can replace one of these attacks with a bite attack. Alternately it can make two attacks with its javelin.

Bite. Melee weapon attack: +9 to hit, reach 5ft, one creature. Hit: 8 (1d6 + 5) piercing damage plus 10 (3d6) poison damage.

Constrict. Melee weapon attack: +9 to hit, reach 10ft, one target. Hit: 12 (2d6 + 5) bludgeoning damage, and the target is grappled (escape DC 17). Until this grapple ends, the target is restrained and the yuan-ti can't constrict another target.

Halberd (Enforcer form only). Melee weapon attack: +9 to hit, reach 15ft, one target. Hit: 21 (3d10 + 5) slashing damage.

Javelin (Enforcer form only). Melee or ranged weapon attack: +9 to hit, reach 10ft or range 30/120ft, one target. Hit: 15 (3d6 + 5) piercing damage if used to make a melee attack or 12 (2d6 + 5) piercing damage and the target must succeed on a DC 16 constitution saving throw or be poisoned until the end of its next turn if used at range.