The information here is somewhat wrong. I acquired all the listed achievements months ago and I didn't get Sequel achievement. I believe you also need to complete the campaign on Legendary and Heroic as well as completing all missions under par time. To put it short, you need every campaign related achievement along with the listed ones to unlock Sequel.--THOU IN HATE SHALL KEEP US BADASS, WHILST WE FIND THE OREO SNACK 12:18, March 3, 2010 (UTC)