In alchemy, a symbol of the sacred fire (sulfur) trapped in the ego, which is poison (arsenic) to the soul.
"We must annihilate the psychic aggregates and the Arsenic Sulfur. The arsenic, poisonous Sulfur is an infernal type of fire within the lower animal depths of the human being. The Arsenic Sulfur is that horrible Python serpent that slithered in the mud of the earth and that the irritated Apollo hurt with his darts. It is the tail of Satan." - Samael Aun Weor, The Gnostic Bible: The Pistis Sophia Unveiled