Raerek's Inscribed Amulet of Talos is an item found during the Stormcloak quest Compelling Tribute. The amulet is used as blackmail leverage as it represents Raerek's belief and worship of Talos. Being the uncle and Steward of Jarl Igmund of Markarth, this could ruin Raerek's reputation as well as destroy the Jarl's family name. As they were to renounce their belief in Talos per the Thalmor as part of the agreement to return to Markarth and the thone, this is indeed a particularely damning piece of evidence.