Complex sentence: At the end of the season, Paisley announced that he would be stepping down as manager; he had won six League championships, three European Cups and League Cups during his reign, the most successful manager in the club's history.
Split into simpler sentences: At the end of the season, Paisley announced that he would be stepping down as manager. He had won six League championships, three European Cups and League Cups during his reign to become the most successful manager in the club's history.