Complex sentence: Columba remained a major figure into the fourteenth century and a new foundation was endowed by William I (r.1165-1214) at Arbroath Abbey and his relics, contained in the Monymusk Reliquary, were handed over to the Abbot's care.
Split into simpler sentences: Columba remained a major figure into the fourteenth century and a new foundation was endowed by William I (r.1165-1214) at Arbroath Abbey. His relics, contained in the Monymusk Reliquary, were handed over to the Abbot's care.