In a passage of the first book of his Dogmatic History, George Metochites (ca 1250-1328) meticulously describes a letter from Emperor Manuel I Komnenos (1143-1180) to the Pope on the subject of the union of the Churches, a letter – written with golden ink on a purple parchment – that the author claims to have seen in the city of Benevento, during a diplomatic mission to the Papal Court. This article presents the first critical edition and an Italian translation of the above-mentioned passage of Metochites’ History and sheds more light on the historical context of the embassies sent by Manuel I to the Pope in the late 1160s: by referring to the testimony of Niketas Choniates’ History and the Vita Alexandri III (Liber pontificalis), it is possible to establish to which of these missions the letter evoked by Metochites should be linked to. Furthermore, this contribution aims to reconstruct both the material aspect and the content of this lost document, by comparing Metochites’ description with three original Auslandsschreiben of the Comnenian period preserved in the Vatican Secret Archive.