Nearly two years since General Mills' last limited-edition launch of Marshmallow-Only Lucky Charms, the brand is bringing back the crowd favorite contest—on an even larger scale. The promotion, which first began in 2015 with the release of just 10 boxes, has continued to grow. In 2017, 10,000 winners received the cereal and now, 15,000 will score a box of the rainbow and unicorn-shaped 'mallows.