Complex sentence: Dallet (Vol 1 pages 28--9) says that Yi Byeok was put under intense pressure from his father and finally more or less gave up the faith, after which he was tormented by remorse until he died of the plague in 1786.
Split into simpler sentences: Dallet (Vol 1 pages 28--9) says that Yi Byeok was put under intense pressure from his father, until finally, more or less, he gave up the faith. He was tormented by remorse until he died of the plague in 1786.