Cadwalla and Penda rose against them, and Edwin fell in battle at Hatfield Moor in Yorkshire. Paulinus, despairing of the cause, returned to Kent with the queen-widow Ethelburga and her children; and under Cadwalla and Penda, the kingdom soon relapsed into Paganism. We must now direct our attention to a small, barren island on the west coast of Scotland, Iona. Here came a voluntary exile (A.D. 563), Columba, a monk, said to have been a descendant of the Irish kings. Here he lived and founded a great missionary monastery, which afterwards became the centre of Christian influence in Scotland and the north of England. He and his followers were active workers; they wrote Gospels and devotional books, preached, and built churches of wood. Columba died (A.D. 597), but his work was continued. In 634, Oswald, a son of Ethelfrith, became king of Northumbria. In his youth he, with his brothers, had been obliged to flee to Scotland, where, during his exile, Oswald was converted to Christianity by the teachers of Iona. On his return he defeated and killed Cadwalla at Hevenfeld, or Heavenfield, near Hexham, in 634, and became the means of finally introducing Christianity into his kingdom. Soon after he became king, Oswald sent to Iona for help, and in reply came a monk, who, for some reason, said by old writers to be his harshness, failed in his mission. He was replaced by another monk named Aidan (635-651), who was eminently successful. Beda speaks of him as "a man of great piety and zeal, combined with tender charity and gentleness." Aidan became intimately associated with King Oswald, the two working together, and he chose for his headquarters the small sandy island of Lindisfarne, off the Northumbrian coast, which we now know as "Holy Island." Lindisfarne thus resembled Iona, and it is probable that the similarity of position and surroundings influenced Aidan in his choice. However that may be, Aidan there founded his monastery and directed the work of his monks.