In 2003 they released a single "Nightingale" as a forerunner of their 2004 album "Insidious" on Fossil Dungeon records.
The cd version of this release had printed in the inlay and on the cd itself this symbol.
The first is of course the symbol of planet Mercury but also of the element, an ancient symbol for what the alchemists refer to as First Matter. It has been very often been used in goth band imagery. See it here along other planetary glyphs.