Jack's motive is to kill Bod, and he succeeded in killing Bod's family. He wants to kill Bod because there is a prophecy that Jack, and the four other Jacks who make up an ancient brotherhood, will face their end if Bod lives to be a man. Their brotherhood is called "Jacks of All Trades, or the Knaves, or by other names", and their power comes from killing people. The brotherhood thinks of Jack Frost as their strongest member, and so they send him to do the job of killing Bod so the prophecy can't be realized. It is also revealed that Silas, who escorts Jack from the graveyard when Bod is a baby, has been fighting the Jacks for a very long time and has contributed significantly to the prophecy coming true.