Blood's Honor

You have been told Kodlak is looking for you. He desires to be free of the curse so that he may enter Sovngarde like a true Nord rather than Hircine's Hunting Grounds. You need to bring him the head of the witches that gave The Companions the Beast Blood to help him.

Speak to Kodlak Whitemane
When you have done enough Radiant quests for the Circle, one of them will tell you that Kodlak is looking for you. He can usually be found in his quarters in Jorrvaskr. Head downstairs to look for him and he will explain your quest to you and give a little history lesson.

You are to go to Glenmoril Coven to get the head of a Glenmoril Witch in order to cure him of the Beast Blood.

Collect a Glenmoril Witch's Head
Go to Glenmoril Coven and enter. There is a witch in the big cavern right at the start. Just kill her, loot the body and take her head. Now you can either go back to Jorrvaskr or proceed to kill the other 4 witches.

Wipe out the Glenmoril Witches (Optional)
If you decided to complete the optional objective, just head into each of the narrow side passages to find the witches. There are some Skeevers and Frostbite Spiders around the place along with the witches. Try to kill them quietly to avoid drawing attention. Once all the witches are dead and all the loot has been taken (along with a spare head to cure yourself if you want), go back to Jorrvaskr.

Note: Only one Glenmoril Witch Head is required for Kodlak, but should the Dragonborn ever what to cure lycanthropy, or want to help Farkas and Vilkas in Purity collecting at least four heads saves some time.

Other than potentially cure the Dragonborn, wiping out all the witches is worth nothing, since you do not get a reward at the end of the quest.

Return to Jorrvaskr
When you arrive, you find the place in disarray with members of The Companions standing outside with dead Silver Hand all over the place. As you head towards Jorrvaskr, Vilkas will stop you and ask where you were. Tell him you were out doing a job for Kodlak and he will tell you that Kodlak was killed in the attack. After he grieves for a while, he will ask you to accompany him to lead a counter attack on the Silver Hand.

Die Ehre des Blutes