In Volsungsaga, Signy marries Siggeir. Volsung and Sigmund were attending the wedding feast (which lasted for some time before and after the marriage), when Odin, in the guise of a beggar, placed a sword into a firebrand or flaming log. He announced that the man who could remove the sword would have it as a gift. Only Sigmund was able to free the sword.
Later, Siggeir and his forces killed Volsung in battle, and Sigmund, with his brothers, were put in stocks in the wilderness, where each night a she-wolf would come and devour one of the brothers. Before Sigmund was eaten, Signy had honey smeared over him and placed in his mouth. When the wolf came to devour Sigmund, she began licking the honey from his mouth, and Sigmund bit off the wolf's tongue, killing her.
Sigmund escaped his bonds and lived underground in the wilderness on Siggeir's lands. While he was in hiding, Signy came to him in the guise of a sorceress and conceived a child by him (Sinfjotl). Bent on revenge for their father's death, Signy sent her sons to Sigmund in the wilderness, one by one, to be tested. As each failed, Signy urged Sigmund to kill them. Finally, Sinfjotl (born of the incest between Signy and Sigmund) passed the test.
Sigmund and his son/nephew, Sinfjotl, grew wealthy as outlaws. As they were killing, they came upon men sleeping in cursed wolf skins. Upon killing the men and wearing the wolf skins, Sigmund and Sinfjotl were cursed to a type of lycanthropy. Eventually, Sinfjotl and Sigmund avenged the death of Volsung.
After the death of Signy, Sigmund and Sinfjotl traveled, and Sigmund married and had two sons, one of them named Helgi. Helgi and Sinfjotl ruled a kingdom jointly. Helgi married a woman named Sigrun after killing her father. Separately, Sinfjotl killed Sigrun's brother in battle, and she avenged him by poisoning Sinfjotl.
Later, Sigmund married a woman named Hjordis. After a short time of peace, Sigmund's lands were attacked by King Lyngi. While in battle, Sigmund matched up against an old man (Odin in disguise). Odin shattered Sigmund's sword, and Sigmund fell at the hands of others. Dying, Sigmund told Hjordis that she was pregnant and that her son would one day make a great weapon out of the fragments of his sword. That son was Sigurd.
Sigmund's story is based on older material than that found in the Sigurd story, and as such it is more directly involved in matters of family descent and the conquest of lands. If there is a historical person behind the Sigmund stories, it is probably a chieftain from the time of the first great Germanic migration in the second and third centuries, A.D.
Analogs for Sigmund's pulling the sword from the fire log can be found in other mythologies (notably in the Arthurian legends), and the "Volsungs" appear tangentially in other Germanic stories (as, for example, "Waelsigs" in Anglo-Saxon literature). Additionally, the device of broken sword that is recast was drawn from this story (and from the Germanic Gotterdamerung) and used by J.R.R. Tolkein for is Lord of the Rings.