Yanko Hristov. "Written Not with Ink but with Tears: Byzantine Civilians in Bulgarian Captivity According to the Letters of Patriarch Nicholas I Mystikos (901–907, 912–925)." Mediaevalia, volume 43 (2022).

In the dynamic situation of the Byzantino-Bulgarian conflict that lasted from 913 to 927, alongside the crucial notices of military successes and failures, of no less importance is the information preserved about the intense diplomatic activity of the era, which reveals the true dimensions of the rivalry between the two biggest powers in Southeastern Europe. In this regard, the letters of Patriarch Nicholas I Mystikos deserve increased attention from scholars for the light they shed on dealings between the Empire and Bulgaria during this period. A year into his second tenure as Patriarch of Constantinople in 912, due to the rapid, successive deaths of Emperor Leo VI "the Wise" (r. 886–912; d. May 11, 912) and his brother, Alexander (r. 912–913; d. June 6, 913), and due to the youth of the heir to the throne Constantine VII Porphyrogennetos (r. 913/945–959; d. November 9, 959), born in May 905, Nicholas I Mystikos proved to be a key member of the Regency for the child emperor. It was from his position as a leading regent that he wrote to the Bulgarian ruler, Tsar Symeon I "the Great" (r. 893–927; d. May 27, 927), in order to prevent his campaign against Byzantium.