Armor Class 16 (natural armor)
Hit Points 189 (18d12 + 72)
Speed 40 ft., swim 30 ft., climb 30 ft.
Skills Perception +7, Stealth +5
Damage Resistances acid, fire, lightning
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 60 ft., blindsight 30 ft., passive Perception 17
Languages Abyssal, Common, Draconic
Challenge 12
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.
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: +10 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) 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 16). 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: +10 to hit, reach 10 ft., one creature. Hit: 27 (6d6 + 6) piercing damage plus 14 (4d6) poison damage.
Volo's Guide to Monsters, p. 202