The Pendragon Spot 

Wikipedia description: In the first book of the series, The Merchant of Death, Robert "Bobby" Pendragon is living a normal life in Stony Brook, Connecticut, where he is the star of Stony Brook Middle School's basketball team. When he is fourteen, his uncle, Press Tilton, takes him to an abandoned subway station where he and Bobby travel through a portal through space and time called a flume. Bobby soon finds himself in a different territory — possibly a different version of history or a different world entirely — Denduron, which is on the brink of a civil war. There Bobby discovers he was chosen to be a Traveler, a defender of the universe from Saint Dane, an evil villain who wishes to tip all of Halla (Everything there ever was, and ever will be) into chaos so he can remake it according to his desire. To save it, Bobby must rescue all ten territories from destruction.