The Jahi is a manifestation of unfulfilled desires of the dead. They concentrate and eventually give rise to a ghostly serpent, which silently seeks out someone to use as its puppet and force them to indulge in all sorts of hedonistic pleasure, and draw others into an orgiastic celebration of excess. To the naked eye, it looks like the person is uncouth and unmodest, though strangely alluring. But those who look closer notice the creature.
This creature is actually based on a demoness of Zoroastrianism. She is a servant Ahriman, the evil side, and seeks out to destroy good with filth and sexuality. She possesses and tempts women and makes them lascivious, and mensturation is a sign of her defilement of them.
My applause to the writers of D&D for taking this creature in an interesting direction. Instead of making the Jahi yet another succubus-like sexy demon lady, they went for something amazingly weird: a multi-headed snake that drains Charisma and controls a host. This is an great monster to have as a bad guy. Though it's Tiny, its challenge rating is a whopping 16. Its touch attack is especially dangerous dealing 1d3 damage and 1d4 Charisma damage (with an extra 5 points of damage to your hp per Charisma taken). Also a neat creature that will probably fight to keep the host it's so carefully been grooming and will probably cut its losses to seek another better host if discovered.