H. P. Blavatsky

Sodalian Oath. The most sacred of all oaths. The penalty of death followed the breaking of the Sodalian oath or pledge. The oath and the Sod (the secret learning) are earlier than the Kabbalah or Tradition, and the ancient Midrashim treated fully of the Mysteries or Sod before they passed into the Zohar. Now they are referred to as the Secret Mysteries of the Thorah, or Law, to break which is fatal.

Source: H. P. Blavatsky, The Theosophical Glossary, art. Sodalian Oath