unfortunately doesn't look like its out on Wii u yet when it does come out, you'll get access to it when you beat the main game, Shovel Knight's campaign just like with Plague Knight

It hasnt been released yet. For now, theres no release date for the wii u version, but YCG is waiting for confirmation from Nintendo of America (and others) about an release date, they believe that soon NoA will give them the confirmation. You can check the "specter knight dlc release date" page on Yacht Club Games site for any new update.

ok, so they updated the website, the dlc will come out 04/21.