Huruf al-Hayy is the central headquarters of the Psi Order ISRA, and the location of the order's Prometheus Chamber. It is conceived to be a space station of some sort, orbiting a celestial body of unknown location. Many have attempted to locate it, with no success.
In reality, Huruf al-Hayy is a secret Lunar colony, hidden miles below the surface of Luna. This fact becomes quickly apparent to new clairsentients upon emerging from the Chamber, as they immediately sense Luna's mass around them. Access is only through four specially crafted access tunnels that have several layers of defenses incorporated into them.