Pope Formosus
Hatto soon became known to the German king, Arnulf, who appointed him archbishop of Mainz in 891, and he became such a trustworthy and loyal 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 favor by Pope Formosus. In 899, when Arnulf died, Hatto became regent of the Empire and guardian of the young king, Louis the Child, whose authority he compelled Zwentibold, duke of Lorraine, an illegitimate son of Arnulf, to recognize.