doubtless he knew much of the doings of hurin 's people and kin , and had not molested them for a while , so that his design might be fulfilled ; but now in pursuit of this purpose he set a close watch on all the passes of the shadowy mountains , so that none might come out of hithlum nor enter it , save at great peril , and the orcs swarmed about the sources of narog and teiglin and the upper waters of sirion .