A famous scene in the Bayeux Tapestry shows Harold, earl of Wessex, swearing an oath on the relics in Bayeux Cathedral. Although the tapestry does not explain the oath, it was generally seen in Normandy as Harold swearing to uphold his promise to King Edward the Confessor and William, duke of Normandy (and later William the Conqueror or William I of England), to support William as successor of Edward.
Picture from Wikimedia Commons.