Keep following the Companion quests to the end to unlock this achievement.
Striking the Heart/Retrieval*** Spoiler - click to reveal ***Aela will send you to strike back at the Silver Hands by assassinating one of their lieutenant in their stronghold. If "Retrieval" is activated instead of "Striking the Heart", you also have to find the Wuuthrad fragment.
Stealing Plans*** Spoiler - click to reveal ***Aela has find out about a Silver Hand lieutenant snooping around Whiterun. He has documents which should lead to his chief. Retrieve the plans.
Blood's Honor*** Spoiler - click to reveal ***Kodlak knows of your revenge campaign, but his researches have led to a cure for lycanthropy. You have to decapitate a Glenmoril witch. I recommend you clear the whole lair and get two witch heads for future use; there are also useful items around. When you return, Kodlak has been killed.
Purity of revenge
Glory of the dead*** Spoiler - click to reveal ***Attend Kodlak funeral, get the fragment in his drawer and give it to Eorlund. Participate in the circle meeting and Eorlund will return with Wuuthrad reforged and the journey to Ysgramor's tomb starts. You have to return Wuuthrad to Ysgramor to open the tomb. Once you make it to the depths of the tomb, you will meet Kodlak ghostly remains. Throw a witch head into the flames and Kodlak bestial spirit will be released. Fight it so Kodlak can enter Sovngard. He will then name you Harbringer of the companions.