The Eye of Fire was the ancient alchemical symbol for fire and destruction. It resembled an "X" inside a rectangle.
Uses By The Beast
After Lorne read Cordelia Chase, the information was sucked from his head by Wolfram & Hart. The images from his brain contained the shape of the Eye of Fire. The Beast used this symbol to create the Rain of Fire. It was also the shape of paranormal events, when plotted on a map.