According to Lord Drillemont, it had long been suspected that the heretics used secret tunnels to travel throughout the highlands of Coerthas undetected. In light of the information you overheard, he is convinced that the heretics encamped in Boulder Downs now plan to make use of such tunnels to ambush the allied forces in Snowcloak. He has ordered a contingent of House Durendaire knights to begin searching for the ambushers, but he believes that your presence would do much to bolster their efforts. Travel to Boulder Downs and rendezvous with the House Durendaire knights.
You arrive in Boulder Downs to discover the knights of House Durendaire slain after what must have been a desperate battle with Iceheart's followers. Before you can take stock of the situation, you find yourself surrounded by the surviving heretics, who are plainly hungry for retribution... Though they fight with the passion of true believers, they are no match for you, and soon they too lie motionless in the snow. What now? For want of a better idea, search the heretics' bodies for anything which might offer some insight into their plans.
You rifle through a heretic's knapsack and find a curious parchment with dates and times writ upon it. Could it be of significance? Lord Drillemont will know.
Lord Drillemont recognizes the parchment as a timetable of guardsmen's shift changes from the outpost in Snowcloak. With it, the heretics could pass into and out of their tunnels without fear of discovery, and thereby prepare an ambush for the allied forces searching the area. Despite your best efforts, it would seem the Temple Knights and Crystal Braves are still in danger, and Lord Drillemont has no choice but to send reinforcements.