Rollo, also known with his epithet, Rollo "the Walker", was a Viking who, as Count of Rouen, became the first ruler of Normandy, a region in today's northern France. He emerged as a war leader among the Norsemen who had secured a permanent foothold on Frankish soil in the valley of the lower Seine. He was a prominent figure among the Vikings who besieged Paris in 885 and led the ill-fated Siege of Chartres in 911. The latter was nonetheless the catalyst for the consequential Treaty of Saint-Clair-sur-Epte, which saw Charles the Simple, king of West Francia, grant Rollo lands between the river Epte and the sea in exchange for Rollo agreeing to end his brigandage, swear allegiance to Charles, convert to Christianity, and pledge to defend the Seine estuary from other Viking raiders.