Crucified Serpent (Alchemical Cross)

The symbol of a crucified serpent is an old alchemical drawing representing the “fixing of the volatile,” or, making the elixir of mercury, a legendary curative, by removing the ‘volatile’ or poisonous element. The picture is derived from the biblical story of Moses, who erected a brazen snake as a charm against plague.
The crucified snake, also known as the Flamel, or the staff of Flamel in alchemical circles follows this meaning, but in mystical terms. The snake in alchemy represents poison, so the crucified snake represents removing the volatile aspects of the potion the alchemist was working with in order to create the fabled "elixir of mercury" also known as the elixir of long life, or immortality. In alchemy, though, all the material tools are symbolic of oneself, so the poison is actually the base and animalistic instincts of oneself that as an alchemist one must remove, or "crucify", in order to accomplish The Great Work.
Another interpretation of the crucified serpent comes from the Gnostic sect of Christianity. The ancient Gnostics believed that the old testament god, who they called the demiurge, was actually evil, and was trying to keep mankind from reaching salvation.