Avoid splicing this cable. It CAN be done but not by a homeowner.
Can you notch a stud or two to reroute it? If so, be sure to put nail plates over the notches to keep screws/nails from damaging it.
If you insist on cutting it, remember that J box must be accessible (easily accessible please) and connections must be made with the appropriate connectors.
Use a big (6x6x4) jbox (I'd use plastic) and some kind of mechanical connector made by Polaris or Ilsco.