Each of these missions represents a test of skill, based on some remembered episode from Cel's past- most will need you to acquire a particular skill before you can essay the challenge. Completing the victory condition for each grants a new skill point. Each mission will also have two special objectives- these will yield mirian and a gem if fulfilled. Complete all the challenges in a region to unlock the memory of a special warchief Cel tangled with long ago. These guys are in the level 40 range, so approach with due caution. Complete all the challenges (with or without the special objectives) to unlock the Legendary item Bright Lord's Rune.