Notice! This mission becomes available only after you join the Thieves Guild during Taking Care of Business (see the guide to The Elder Scrolls V: Skyrim - College of Winterhold and Thieves Guild). You also need to have the Thieves Guild armor and speak with Delvin Mallory and Vex.
This mission has been prepared with the members of the Thieves Guild in mind. In order to start it, speak with the blacksmith at Raven Rock - Glover Mallory. The man will tell you that his former partner stole his recipe for the Improved Bonemold Armor. Such insult cannot go unpunished!
Glover's apprentice, Esmond Tyn, can be found far in the north, in the Castle Karstaag Caverns. Getting there isn't easy - the caves have to be approached from the south (through the mountains). By the waterfall you should find a small trough. Inside there's a whole army of Rieklings, while on the ice island right by the entrance - the corpse of the poor man. Search his corpse to find the bonemold formula - you can return to the quest giver.
Bring the Bonemold Formula to Glover Mallory
Return to Raven Rock and speak with Glover Mallory. The man will be sad to hear that his former partner is dead and will take back the formula. In return you will receive the key to his basement - inside you can find tons of useful items. It was a really lucrative mission!