The Deathsingers Ir Anuk and Ir Halak are not awarding their respective Grimoire cards. I've killed them three times in the King's Fall Raid and haven't gotten them either time. Ive also checked the Grimoire from others that have beaten the Raid, and they don't have them either. Warpriest, Golgoroth, and Oryx all unlocked just fine. Also, it is impossible to kill them in the Last Rites story mission. Edit: Apparently you get this from killing them in the Last Rites story mission (which doesn't make too much sense...). This is not glitched after all - sorry for jumping the gun.