The promotion to Defender requires you to complete two quests – “The Master’s Son” and “More Unfinished Business”.
The Master’s Son – Talk to Modryn Oreyn in Chorrol and will give you a quest to escort Viranus Donton to Nonwyll Cavern to locate Galtus Previa. Once you locate Viranus, travel to the cavern, north of Chorrol’s North Gate, and investigate. You’ll have to fight a number of monsters, including trolls, so spells or weapons with fire damage can be especially helpful. Fortunately, Viranus is “essential” so he cannot be killed during the course of this quest. You’ll find the body of Galtus Previa in the second part of the cavern. After finding the body, return to Modryn for your reward.
More Unfinished Business – Talk to Modryn and he will tell you that Maglir has defaulted on another mission and sends you to Bravil to find out why. Ask around Bravil until you find out that Maglir is at the Lonely Suitor Lodge. When you talk to him, he’ll reveal that he’s joined the Blackwood Company. Depending on how you handled his earlier defaulted quest he will either tell you about the quest that he’s quit on now or you’ll have to go back to Modryn to find out the details. Either way, you’ll be directed by one of them to talk to Aryarie who needs ten pieces of Imp Gall. Talk to her and she’ll send you to Robber’s Glen Cave. Explore the cave until you’ve killed and harvested ten imps and then return to her. After you’ve turned over the Imp Gall, return to Modryn Oreyn for your reward and promotion.