Complex sentence: Chaosium planned a second edition, with minor changes to the rules, but this was never actually released; they released a third edition, with rules revised by Stafford, as a single softbound book in 1990.
Split into simpler sentences: Chaosium planned a second edition, with minor changes to the rules, but this was never actually released. They released a third edition, with rules revised by Stafford, as a single softbound book in 1990.