Get the quest from glover the blacksmith. There is a shadow mark on his house. You need to have completed 'meet the family' thieves guild quest. Go to a large glacial cave complex full of reiklings, find the quest item (bone mold formula, it's not the reward) and it comes out in the Karstagg fight area. Take it back to Glover and he gives you the key to his basement where you find some level loot and the BlackGuards Armor, along with a note to Sapphire. Talk to her and she gives you a unique exquisite sapphire.