Poland began to form into a recognizable unitary and territorial entity around the middle of the tenth century under the Piast dynasty. Poland's first historically documented ruler, Mieszko I, was baptized in 966, adopting Catholic Christianity as the nation's new official religion, to which the bulk of the population converted in the course of the next centuries. In the twelfth century, Poland fragmented into several smaller states, which were later ravaged by the Mongol armies of the Golden Horde in 1241, 1259 and 1287. In 1320, Władysław I became the King of a reunified Poland. His son, Casimir III, is remembered as one of the greatest Polish kings.