The cover of the comic book 'Once and Future' issue 21, featuring a woman riding a dragon-like creature with a fiery mane and tail. The background is dark with a gradient of red and yellow hues, and the title 'Once and Future' is prominently displayed at the top with the issue number '21' below it. The creators' names, 'Kieron Gillen' and 'Tamra Bonvillain', are visible at the bottom.