Garath told you that the ceremony required relics lost to time. Herka had no information on them, but had you ask Faithful Respondent Utra about them. The Drakan ghost told you that the Spirit's Incense Burner and Scroll of Repose were held by long-dead servants of Tiamat and Stormwing. You talked to them and fulfilled their requests, receiving the Incense Burner and Scroll in return. You took the implements back to Garath, who successfully performed the ceremony and opened up a new Stigma slot for you.