The Symbol of Lilith, also known as Innana in the Sumerian Texts. Lilith - the Femme Fatale of astrology - Lilith is also known as Black Moon Lilith and is represented by a Black Crescent Moon symbol, sometimes atop a cross.
"Black Moon" Lilith is the moonspot in your radix which is most distant from the earth. It means you cannot wear any mask or find ways of protection. Also, on a subconcious level, you are completely naked there, thus most vulnerable.