Complex sentence: The most important rival was published by Tory William Blackwood, Walter Scott's publisher, particularly "Blackwood's Magazine", founded as the "Edinburgh Monthly Magazine" in 1817 and later shortened to "Maga".
Split into simpler sentences: The most important rival was published by Tory William Blackwood, Walter Scott's publisher. It was known as "Blackwood's Magazine", but founded as the "Edinburgh Monthly Magazine" in 1817 and later shortened to "Maga".