Complex sentence: De Courcy's rise to power in the last quarter of the 12th century was breathtakingly swift, although he was eventually ground down by his rivals and John, King of England, in the early 13th century.
Split into simpler sentences: De Courcy's rise to power in the last quarter of the 12th century was breathtakingly swift. He was however ground down by his rivals, and the King of England, in the early 13th century.