After completing a few Companions Subquests and the Companions Mainquest The Silver Hand, speak with Kodlak in the living quarters downstairs of Jorrvaskr. He will inform you that he is aware of the secret attacks that yourself and Aela have been conducting and sheds some light on the curse that plagues the Companions. After you sit and listen to the tale Kodlak has to offer, he tasks you with the retrieval of a Glenmoril Witch’s head. Make your way over to the Glenmoril Coven the best way you know how and proceed to kill the witches located within.
Note: Although only one head is needed, it is advised that you kill all five Glenmoril Witches located here so that you will not need to make future return trips to this location for other quests that are soon to come.
Once you have at least one Witch head, travel back to Whiterun and hurry back to Jorrvaskr to discover that the Companions were attacked by the Silver Hand as retaliation for the attacks that Aela sent you on. Regrettably, a few Companions have fallen, including Kodlak Whitemane.
At this point in time the next Companions main quest Purity of Revenge begins and all subquests are locked until you complete the Companions story line of quests.