The Leviathan Cross is sometimes refered to as Crux Satana, which means the Devil's Cross. Depicted on the bottom is an infinity sign (∞), and above is a double cross (‡). The Double cross symbolizes protection and balance between male and female. The infinty sign most likely means eternal rest/hapiness, but may have been used by LaVey as a symbol for eternal damnation.
In Alchemy, the Leviathan's cross is a symbol for sulfur, which is one the three essential elements of nature.
The origins of the Leviathan Cross is created/used by The Knights Templar. The symbols was then adopted by Anton LaVey, founder of the church of Satan. Previous associations with the cross and satanic foundations are unable to be identified.