Tothwah

Originally Tothwah was the minor demi-god of darkness. But through her ambition, willpower and ingenuity, she succeeded, through the cover of the darkness that gives her extra powers, to corrupt a large proportion of the elves and turn them into Drows. In doing so, she earned herself the status of full godhood.
In Drow culture she is most often depicted as being an elf-spider hybrid. With an elf torso mounted on a spider's body. And the holy symbol is most often a variation of this theme.
The teachings of Tothwah permeate Drow culture and anything associated with drows most often stems from Tothwah. Anything you can learn about drows and their culture has some kind of root in the worship of Tothwah. She promised these elves incredible powers in the darkness if they followed her teachings. And she delivered on her promise.
It has some dire consequences though and drow culture is a very unforgiving and harsh one. Babies who are not an advancement of the race are often tossed aside and left to die as only the strongest are to survive to increase the strength of their species. 
Tothwah also finds men inferior and hence the drow society is a very matriarchal one. No male drow can hold any position of power and Tothwah only allows female priests and clerics in her church.