Envoy To Constantinople
In 869 he was sent by Louis II, Holy Roman Emperor as envoy to Constantinople, with two men of high rank in the Frankish Empire, to negotiate a marriage between Leo VI the Wise, oldest son of the Byzantine emperor Basil I, and Louis's only child, Ermengard.
When the envoys arrived at Constantinople, the Fourth Council of Constantinople was still in session, and Anastasius, who attended the last session (February, 870), zealously defended the papal cause and was of much service to the papal legates.
On their way home the papal legates were robbed, and the "Acts" of the council were carried off. However, they had given most of the declarations of obedience of the Greek bishops to Anastasius, who also had a copy of the "Acts", and was thus able to bring these documents to the pope. At the pope's order, he translated the "Acts" into Latin. The Greek originals are lost.
Read more about this topic: Anastasius Bibliothecarius