While Satan has been the 'Accuser of the Brethren' for ages, the reference is made to the Jewish Remnant (the brethren of Christ) who will go through great persecution during the first 3 1/2 years of the Tribulation Period and will die as martyrs.

           They are the 'souls of them  that were slain for the Word of God,' and they overcome by the 'Blood of the Lamb' and the 'Word of their Testimony' and as martyrs they 'loved not their lives unto the death.'