Building on the archetypal stories of the New Testament and the early church fathers, an orthodox tradition of demonology emerged from the fourth century in the Greek Christian literature of the Eastern Roman Empire (now commonly known as Byzantium). Here the demons, as fallen angels, preserved aspects of their former nature, such as immortality and incorporeality, but they had many limitations. Having lost their natural luminance and goodness, they had become completely evil, embodying all negative Byzantine cultural values. And they were thought to be dangerous. Lurking invisibly everywhere and possessing no individual identity or shape of their own, demons could transform their appearance at will to tempt or possess their human victims, luring them into actions that would jeopardize the salvation of their souls. At the same time, however, alongside these conceptions, other understandings of demons persisted and evolved in popular thought. Here some sort of physicality was ascribed to them, and a vast array of specific types was identified, based on their particular functions and where they were thought to live. One tradition of this type, which persisted throughout the long span of Byzantine history and continued in Greek folk belief into the modern era, was that of a specifically female demon, known by a variety of names, but perhaps most commonly as the Gellou, who was blamed for the illness and death of newborns and children.