Well, it's the symbol of Tharizdun, in RttoEE. But I would like to know what it does look like. We have defeated those clerics, and I can't visualize their unholy symbol.
The holy symbol of WG4 is an "inverted, two-tiered black pyramid" "with bases to the north, points to the south" (p. 14,15), called an "obex" by some. This is illustrated on the monks' cloaks on the internal cover (p. 1), albeit in a stylised fashion. The symbol on this page's background is truncated such that it matches the shape of the Temple (see below). According to Gygax (Oerth Journal #12), the cult chooses its symbol based on where it perceives darkness. Right now, it's under the Temple; in former days they adopted the sign of the black sun.