Aela has learned that the Silver Hand are holding a fragment of Wuuthrad at Fort Fellhammer, which of course means we're going to retrieve it. That fragment shall be ours!
Our target, Fort Fellhammer, is just south of the city of Dawnstar. Head to Dawnstar using a carriage or simply walk there, and head south from there. You're likely to run into wolves or ice essence enemies on your way.
The outside of the fort itself--which like all forts has no gate--is guarded by three Silver Hand members, likely ready to take pot-shots at you with their bows. Take them out and proceed to the second level of the fort so that you can enter The Garrison .
Inside there's only one enemy: the Silver Hand Leader. He's probably sitting alone in a room, looking away from you, giving you a perfect chance to sneak attack him (for 3x damage too - sweet!). Take him out however you want and check the chest for the fragment of Wuuthrad.
The leader is also wearing some really strong Nordic Carved Heavy Armor too, so if you like this type of armor, now's a great chance to pick some good quality gear for free.
Well, that was easy! Now our job is to return to Aela and give her the fragment. Once you do that you'll be rewarded with 300 Gold and this quest will be over, replaced by the next quest: Striking the Heart .