Card-Jitsu Shadow is a possible future game in Club Penguin, which could be the final part of the Card-Jitsu Series. There are some clues that show it might be available in the future.
On his Twitter account, Spike Hike got a tweet asking if there would be a Card Jitsu Shadow. He replied, "Snow first." which likely proves that Card-Jitsu Shadow is coming out. When also asked whether it is real, he said it is a rumor but he believes in it. 
On a Club Penguin Memories post of Ask Spike Hike. Spike Hike mentioned that there world be something big for Card-Jitsu. He may have been referring to Card-Jitsu Shadow. He also mentioned something big for Puffles.
When asked, Sensei said "There might be Shadow some day," giving more evidence that it may be released but not giving the exact date. Another time at the Card-Jitsu Party 2013, Sensei replied to a similar question, responding "One element at a time. First we must master Snow to restore balance", giving even more hint that sometime in the future, Card-Jitsu Shadow might happen.
While talking about the dark side of the force, Obi-Wan Sensei said Tusk had turned to the shadows, a possible reference to Card-Jitsu Shadow.