Complex sentence: The book also contained an overview of the Known World (Mystara) and Hollow World campaign settings, as well as rules on how to convert characters between the "Dungeons & Dragons" game and the "Advanced Dungeons & Dragons" second edition game.
Split into simpler sentences: The book also contained an overview of the Known World (Mystara) and Hollow World campaign settings. It also has rules on how to convert characters between the "Dungeons & Dragons" game and the "Advanced Dungeons & Dragons" second edition game.