Just find and slay six Summit bosses. The map will not point the way to the bosses, but if you did the quest Summit Slaves, you will find bosses in the exact same spawn locations. The gates to the Forge offer confirmed boss spawns but they are also the hardest bosses to kill because of the Furnace Guardians. Tip: Bosses killed on the same trip into Sorrow's Furnace before receiving this quest do count towards your total. It is possible to kill six bosses, speak to Tharn to receive the quest, then immediately speak to him again to receive the quest reward.