Complex sentence: In 1991, CMLL decided to create a series of World Championships, one of which was the for the Middleweight division, using a one-night, eight-man tournament to determine the first Middleweight Champion.
Split into simpler sentences: In 1991, CMLL decided to create a series of World Championships, one of which was the for the Middleweight division. They use a one night, eight man tournament to determine the first Middleweight Champion.