About the time of the accession of Leo VI's brother Alexander to the throne in May 912, Nicholas was restored to the patriarchate. A protracted struggle with the supporters of Euthymios followed, which did not end until the new Emperor Romanos I Lekapenos promulgated the "Tomos of Union" in 920. In the meantime Alexander had died in 913 after provoking a war with Bulgaria, and the underage Constantine VII succeeded to the throne. Nicholas Mystikos became the leading member of the seven-man regency for the young emperor, and as such had to face the advance of Simeon I of Bulgaria on Constantinople. Nicholas negotiated a peaceful settlement, crowned Simeon emperor of the Bulgarians in a makeshift ceremony outside Constantinople, and arranged for the marriage of Simeon's daughter to Constantine VII.