"The Revenge of the Shadow King" by Derek Benz & J. S. Lewis

Can the Grey Griffins save their town from the evil Shadow King?

Max Sumner and his three best friends, Harley, Ernie, and Natalia--who form the secret club The Grey Griffins--seem to be the only people in their very normal Minnesota town to notice that strange things have started to happen. When creatures like goblins and fairies and unicorns, all characters from a card game the Grey Griffins play, begin to make appearances in Max's backyard, Max and his friends know something is terribly wrong. And it's up to them to stop the wicked creatures of the cards from destroying their town-indeed, their world.