Chameleon Skin. The anathema has advantage on Dexterity (Stealth) checks made to hide.

Innate Spellcasting (Anathema Form Only). The anathema’s innate spellcasting ability is Charisma (spell save DC 18). 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

Legendary Resistance (3/Day). If the anethema fails a saving throw, it can choose to succeed instead.

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 18 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.

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

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

Claw (Anathema Form Only). Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) slashing damage.

Constrict. Melee Weapon Attack: +11 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.

Flurry of Bites. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 27 (6d6 + 6) piercing damage plus 14 (4d6) poison damage.

Acid Slime. The anathema coats its body in a slimy acid that lasts for 1 minute. A creature that touches the anathema, hits it with a melee attack while within 5 feet of it, or is hit by its constrict attack takes 5 (1d10) acid damage.

Change Shape. The anathema polymorphs 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.

Shed Skin (Recharges after a Long Rest). The anethema sheds its skin to free itself from a grapple, shackles, or other restraints. If the anathema spends 1 minute eating its shed skin, it regains 105 hit points.

The anathema 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 anathema regains spent legendary actions at the start of its turn.

Claw. The anathema makes one claw attack.

Serpent's Coil (Costs 2 Actions). The anathema moves up to its speed and attacks with its flurry of bites.

Cast Spell (Costs 3 Actions). The anathema casts one of its innate spells.