Do you believe Satan is a very real deity, or is Satan just a symbol? It seems to me that monotheism would require the latter, but what's your viewpoint?
Feel free to include other "dark lords" and religious text if you want.
Satan is referred as "the god of this system of things" at 2 Corinthians 4:4. Jesus called him the ruler of this world. At John 14:30 Jesus said; "the ruler of the world is coming, and he has no hold on me." I believe Satan is an apostate angel who rebelled against the true God. Jesus said about Satan; "That one was a murderer when he began, and he did not stand fast in the truth, because truth is not in him. When he speaks the lie, he speaks according to his own disposition, because he is a liar and the father of the lie." (John 8:44) Satan's destruction is certain, according to the Bible.(Hebrews 2:14)