Although sidelined after Theophilos's death by Theodora and Theoktistos, in 855 he engineered Theoktistos's murder and became the de facto regent for his nephew, Michael III (r. 842–867). Rising to the rank of Caesar, he was the effective ruler of the Byzantine Empire for ten years, a period which saw military success, renewed diplomatic and missionary activity, and an intellectual revival that heralded the Macedonian Renaissance. He was assassinated in 866 at the instigation of Michael III's new favourite, Basil the Macedonian, who a year later would usurp the throne for himself and install his own dynasty on the Byzantine throne.