The Crossroads is a wrinkle in spacetime where myriad parallel worlds meet.
Sometimes the inhabitants of one world cross into another. People from our world may travel into the world next door and find that it's just a little different from ours. They may be visited by denizens of a more distant world with greater differences. People of any world may sometimes meet creatures so alien that their worlds must be very different indeed.
Sometimes visitors bring with them traces of the natural laws of their home worlds. When these traces cause strange effects that we don't understand, we call them "magic."