The Shadows of the Past missions are the missions in which you play as Celebrimbor in a memory of his. Most missions require a certain skill to be unlocked prior to it being playable. On a region map the Shadows of the Past missions will be represented by a helmet icon. Once a mission is completed it will become accessible in the region's Barrow. Just knock out all the missions in one region, rating earned on each mission does not matter, and the achievement will pop.