The black sun indicates the idea of the world going wrong, disastrous events and death. The symbol above, called the Sonnenrad, or Sun Wheel, was based on the design of early medieval broaches, some of which had a swastika in the center.
The rays numbered 5 to 12, denoting the passage of the Sun through the months of the year.
The Black Sun was used by wartime German occult mystics and is still employed by some neo-nazis.
In alchemy, it is believed that there are two Suns: one of pure “philosophical gold”, and the other the base “material gold.” The Black Sun symbolizes the unworked matter that needs to be developed.