HATTO I. (c. 850-913), archbishop of Mainz, belonged to a Swabiah family, and was probably educated at the monastery of Reichenau, of which be became abbot in 888. He soon became known to the German king, Arnulf, who appointed him archbishop of Mainz in 891; and he became such a trustworthy and confidential counsellor that he was popularly called " the heart of the king." He presided over the important synod at Tribur in 895, and accompanied the king to Italy in 894 and 895, where he was received with great favour by Pope Formosus. In 899, when Arnulf died, Hatto became regent of Germany, and guardian of the young king, Louis the Child, whose authority he compelled Zwentibold, king of Lorraine, an illegitimate son of Arnulf, to recognize. During these years he did not neglect his own interests, for in 896 he secured for himself the abbey of Ellwangen and in 898 that of Lorsch . He assisted the Franconian family of the Conradines in its feud with the Babenbergs, and was accused of betraying Adalbert, count of Babenberg, to death. He retained his influence during the whole of the reign of Louis; and on the king's death in 911 was prominent in securing the election of Conrad, duke of Fran'conia, to the vacant throne. When trouble arose between Conrad and Henry, duke of Saxony, afterwards King Henry the Fowler, the attitude of Conrad was ascribed by the Saxons to the influence of Hatto, who wished to prevent Henry from securing authority in Thuringia, where the see of Mainz had extensive possessions. He was accused of complicity in a plot to murder Duke Henry, who in return ravaged the archiepiscopal lands in Saxony and Thuringia. He died on the 15th of May 913, one tradition saying he was struck by lightning, and another that he was thrown alive by the devil into the crater of Mount Etna. His. memory was long regarded in Saxony with great abhorrence, and stories of cruelty and treachery gathered round his name. The legend of the Mouse Tower at Bingen is connected with Hatto II., who was archbishop of Mainz from 968 to 970. This Hatto built the church of St George on the island of Reichenau, was generous to the see of Mainz and to the abbeys of Fulda and Reichenau, and was a patron of the chronicler Regino, abbot of Priim.
See E. Dummler, Geschichte des ostfrankischen Reichs (Leipzig, 1887-1888); G. Phillips, Die grosse Synode von Tribur (Vienna, 1865) ; J. Heidemann, Hatto I., Erzbischof von Mainz (Berlin, 1865) ; G. Waitz, Jahrbucher der deutschen Geschichte unter Heinrich I. (Berlin and Leipzig, 1863); and J. F. Bohmer, Regesta archiepiscoporum Maguntinensium, edited by C. Will (Innsbruck, 1877-1886).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)