Sirens wander about on a sacred pilgrimage to spread the gospel of their Elder Sisters.
They are giant pale, thin, bony, and rotten humanoids. From the collarbones and up is an entire steel-frame signal tower. They are always "singing," broadcasting an unfathomable signal. Humans within range of this signal experience hallucinations that induce extreme fear, and if they do not manage to escape in time, their consciousness is destroyed and they are reduced to a shell of their former selves devoid of any intelligence or consciousness. However, there are rumors that these afflicted humans have heard information that was not meant for human ears.
