An ancient Aragonese legend tells how, at the end of the 11th century, two French priests crossed the Pyrenees bearing the remains of an old eastern martyr, Demetrius, with them, loaded on a pack mule. The animal was blind and did not stop walking until it fell dead at the foot of Loarre castle, the fortress from which King Sancho Ramírez launched campaigns to extend the Kingdom of Aragón at the expense of the Muslims. The reliquary we know as the ‘Arqueta’ of Saint Demetrius is a Romanesque metal box dated to approximately the year 1100. However, the presence of another Byzantine piece, a Constantinopolitan ivory from the 10th century, also related to the royal couple in Aragón during the same years, makes us ask whether the Romanesque reliquary casket originally contained the relics. The simultaneous presence of both the relics, and an exquisite ivory fragment from the Eastern Mediterranean in a small kingdom located at the western end of Europe is indeed curious. In this paper I thus aim to analyse the ‘opportune and convenient’ arrival of the remains of the Thessalonican saint in Aragón, and its possible role in the religious and political programme defining the reign of Sancho Ramírez, as well as to try to formulate some hypotheses regarding what might have happened to the original reliquary of Saint Demetrius.