In Dragon #151, the (Oriental) yuan-ti described there are made to worship a demon lord called Sch'theraqpasstt, who resembles a great, black-scaled winged snake. The article goes on to say that the yuan-ti have turned to other creatures of the Abyss for power since Sch'theraqpasstt's decline. Sch'theraqpasstt is a deity in decline, and is said to have gone mad while attempting to merge with his Abyssal layer.
Desiring to gain ultimate power, it conducted a ceremony designed to completely merge its mind with that of the 111th layer of the Abyss, becoming a sentient, infinite realm. The layer is now called the Mind of Evil, a place where reality itself is so insane and capricious almost no life can survive.