Complex sentence: In 1398 John of Gaunt and Katherine Swynford founded a chantry in the cathedral, to pray for the welfare of their souls, and in the 15th century the building of the cathedral turned to chantry or memorial chapels.
Split into simpler sentences: In 1398 John of Gaunt and Katherine Swynford founded a chantry in the cathedral to pray for the welfare of their souls. In the 15th century the building of the cathedral turned to chantry or memorial chapels.