There is this book whose title I seem to have forgotten.?

There were two main characters, a boy and a girl who lived in a regular world. Then, one day they buy this video game and while playing it, they are sucked into the video game world. There they have to fight with other creatures to stop this guy from becoming a creature with the utmost power in the world. They have to got through many different types of land, and battle many foes. They also have to collect pieces of a certain orb to actually stop the other bad guy. They have a friend who is from that world and is helping them out but in the end he turns out to be the master villain. He actually collects all the pieces of that certain orb and turns into a creature with the utmost power over all human beings. He becomes a cute puppy! I am so sad I forgot the name of this book. If you find out, PLEASE PLEASE, PLEASE let me know.