This group includes the god Njord and his children , Freyr and Freyja . These gods joined the Ã sir as hostages after a war between Ã sir and Vanir . A second clan of gods , the Vanir , is also mentioned in Norse mythology : the god Njord and his children , Freyr and Freyja , are the most prominent Vanir gods who join the Ã sir as hostages after a war between Ã sir and Vanir .