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 anathama'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.

Actions

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.

Spellcasting (Anathema Form Only).

The anathema casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 17):

At will: animal friendship (snakes only)
3/day each: darkness, entangle, fear, polymorph, suggestion

Bonus Actions

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.

Environment: Desert, Forest, Underdark

Yuan-ti Anathema