Countdown #21 was published by DC Comics on December 1, 2007, as part of a series that began in 2007. Keith Giffen and Paul Dini write, and art comes from Hi-Fi, Jamal Igle and Jon Bogdanove. This 36-page issue features Harley Quinn, Donna Troy, Jason Todd, Kyle Rayner and Darkseid, among 32 characters cataloged on VerseDB.