A symbol in alchemy of the sacred fire or Kundalini.
"Sulfur is the fire of Alchemy." - Samael Aun Weor, Treatise of Sexual Alchemy
Since we have lust, pride, envy, etc., that fire is modified, and is called Arsenic (i.e poisonous) Sulfur.
"In these studies of Kabbalah, we need to be practical; there are authors who write marvels, but when one looks at them, one realizes that they have not lived what they have written; they did not experience it in themselves, and that is why they are mistaken. I understand that one must write what one has directly experienced by oneself. I have proceeded in this way for my part."