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

This is the first book in a series called the Grey Griffins series. The Grey Griffins is a group of four eleven-year-olds who are very close friends. The leader, and main character, is Max Sumner who comes from a family that owns towns. Max's parents just got divorced and he doesn't feel like he has anyone to take care of him.

Max, Harley, Natalia and Ernie play a card game called Round Table. It gets really freaky when Max finds a magical book in the attic. It get even more crazy when the creatures in the Round Table game start appearing in Avalon, Minnesota. The Grey Griffins discover a secret world and those that fight against the creatures.

Totally surprising how good this book, and the series, is once you give it a chance.