Sithis (also known as Padomay) is a deity, representative of emptiness and the void. He is worshipped by the Dark Brotherhood. Used to represent him symbolically is a skull, as nothing really comes close to representing the true emptiness of the dread father.
"Hmm... That's like telling you about the cold of space, or terror of midnight. Sithis is all those things. He is... the Void."