Speak to Aela to find out Kodlak is looking for you. When you talk to him, he’ll tell you that the beast form is actually a curse and that to be rid of it, you must collect the heads of the Glenmoril Witches. To do that, he’ll send you off to Glenmoril Coven.
The inside of the Coven is actually pretty simple. The central chamber has a single witch and possibly a couple of skeevers. From the central chamber are four branches, each leading to another room and another witch. Killing the first witch and taking her head will complete the quest. Killing the other four witches is optional and gives you no further reward except that you’ll have the witch’s heads if you wish to get rid of your own beast spirit later on. Just remember to be careful because the witches can be quite powerful. Once you’ve collect your head(s), return to Jorrvaskr.