Innate Spellcasting (Anathema Form Only). The anathema's innate spellcasting ability is Charisma (spell save DC 17). It can innately cast the following spells, requiring no material components:

At will: animal friendship (snakes only)

3/day each: darkness, entangle, fear, haste, suggestion, polymorph

1/day: divine word

Magic Resistance. The anathema has advantage on saving throws against spells and other magical effects.

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.

Shapechanger. The anathema can use its action to polymorph into a Huge giant constrictor 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.

Six Heads. The anathema has advantage on Wisdom (Perception) checks and on saving throws against being blinded. charmed, deafened, frightened, stunned, or knocked unconscious.

Variant: Chameleon Skin. The yuan-ti has advantage on Dexterity (Stealth) checks made to hide.

Multiattack (Anathema Form Only). The anathema makes two claw attacks, one constrict attack, and one Flurry of Bites attack.

Flurry of Bites. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 22 (4d6 + 8) piercing damage. The target must succeed on a DC 19 Constitution saving throw or become poisoned. While poisoned in this way, the target can't regain hit points, and it takes 13 (4d6) poison damage at the start of each of its turns. Only a spell or class feature may cure this poision.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) slashing damage.

Constrict. Melee Weapon Attack: +10 to hit, reach 15 ft., one Large or smaller creature. Hit: 16 (3d6+6) bludgeoning damage plus 7 (2d6) acid damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained and takes 16 (3d6+6) bludgeoning damage plus 7 (2d6) acid damage at the start of each of its turns, and the anathema can't constrict another target.

Variant: Acid Slime. As a bonus action, the yuan-ti can coat its body in a slimy acid that lasts for 1 minute. A creature that touches the yuan-ti, hits it with a melee attack while within 5 feet of it, or is hit by its constrict attack takes 5 (1d10) acid damage.

Variant: Shed Skin (1/Day). The yuan-ti can shed its skin as a bonus action to free itself from a grapple, shackles, or other restraints. If the yuan-ti spends 1 minute eating its shed skin, it regains hit points equal to half its hit point maximum.