Betrayal at the Gates is a side quest in Saradush. To recieve it you will have to enter the Militia headquaters in the city. There you will see a trial where a man named Mateo is sentenced to death for treason on the testimony of Countess Santele. After the trial, you need to speak to Captain Samand and you will learn that he is rather doubtful about the case, and that his initial prime suspect was a man named Kiser Jhaeri. Offer to help and speak with Mateo.