Complex sentence: They had, however, grown so powerful that even such a measure failed to crush them; so numerous were they that a Donatist Council, held at Carthage, in 327, was attended by 270 bishops.
Split into simpler sentences: They had, however, grown so powerful that even such a measure failed to crush them. They were so numerous that a Donatist Council, held at Carthage, in 327, was attended by 270 bishops.