The rokuro-kubi’s innate spellcasting ability is Charisma (spell save DC 12). The rokuro-kubi can innately cast the following spells, requiring no material components:
Magic Resistance. The rokuro-kubi has advantage on saving throws against spells and other magical effects.
Multiattack. The rokuro-kubi makes two attacks, but can constrict only once. Bite. Melee Weapon Attack: +4 to hit, reach 10 ft., one creature. Hit: 4 (1d4 + 2) piercing damage plus 7 (2d6) poison damage. Constrict. Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage, and the target is grappled (escape DC 12). Until this grapple ends, the target is restrained, and the rokuro-kubi can’t constrict another target
Like a number of other creatures like the hannya and the twilight naga, the rokuro-kubi combines features of a serpent with human appearance. The rokuro-kubi’s only snakelike feature, though, is its long neck, which it can coil around an opponent to squeeze or strangle him to death. Under most circumstances, rokuro-kubi are indistinguishable from humans, and they take pains to maintain the illusion of humanity. They typically dress in fine clothes, or else the humble garments of a pilgrim or other traveler. The illusion is quickly shattered when a rokuro-kubi attacks its chosen victim, as it stretches its neck like a huge snake, to a length of up to 10 feet. A rokuro-kubi’s mouth is filled with sharp, serrated fangs.