Vergelmir said that finding the Drana Fragment and the source of the booklet will complete his Stigma research, but the clues were in Dark Poeta. You went to see Hresvelgr, he confirmed Vergelmir's conjectures to be true and told you to go to Dark Poeta. When you returned after obtaining the Drana Fragment and Hewahewa's Scroll in Dark Poeta, Vergelmir increased your Stigma slot by one.