Sinfjotli is the son of Sigmund and Signy, who are brother and sister (see Sigmund and/or Signy for information on his childhood).  Sinfjotli is totally psychotic.  He is so tough that he is impervious to starvation or poison, he can kill a man, woman or child without pausing.

He is raised by his uncle/father until he is old enough to avenge Volsung’s death by killing his mother’s husband.  He and Volsung become great friends and later raid and rule a kingdom together.  Then, “he saw a lovely woman and strongly desired to have her” (Byock 50).  The brother of Borghild, Sigmund’s wife, also wanted to marry the woman, so Sinfjotli killed him.  He then went raiding and became the most renowned Viking of his time.  When he returned and told of the death of Borghild’s brother, Borghild asked Sigmund to expel Sinfjotli from his kingdom, which he would not do. 

So Borghild arranged a funeral feast for her brother, during which she encouraged Sinfjotli to drink ale from a drinking horn that she had poisoned.  Sinfjotli knew it was poisoned, but Sigmund kept taking the horn from him to prove that it was not.  The poison did not affect Sigmund, and he advised Sinfjotli to filter the ale through his mustache.  It didn’t work, though, and Sinfjotli died immediately.  “Sigmund’s…sorrow was almost his death” (Byock 51).