Armor Class 16 (natural armor)
Hit Points 189 (18d12 + 72)
Speed 40 ft., swim 40 ft., climb 40 ft.
Skills Perception +11, Stealth +5
Damage Resistances acid, fire, lightning
Damage Immunities poison
Condition Immunities poisoned
Senses blindsight 30 ft., darkvision 60 ft., passive Perception 21
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 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 anathema's aura for the next 24 hours.
Six Heads. The anathema has advantage on saves against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.
Multiattack (Anathema Form Only). The anathema makes two Claw attacks 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.
Flurry of Bites (Anathema Form Only). Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. Hit: 27 (6d6 + 6) piercing damage plus 14 (4d6) poison damage.
Constrict (Snake Form Only). 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 it takes 16 (3d6 + 6) bludgeoning damage plus 7 (2d6) acid damage at the start of each of its turns. The anathema can constrict only one creature at a time.
Change Shape. The anathema transforms into a Huge 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.
Mordenkainen Presents: Monsters of the Multiverse, p. 272