I'm not sure that you need a mystery ball. As long as you can bowl the normal off spinner and arm ball you should be able to take plenty of wickets by varying the flight, loop, length and turn. But you must have appropriate field placements to take advantage of a batsman's mistake. Please read the info in the following two links which should help you understand the field placements for an off spinner.

http://www.rediff.com/cricket/2002/aug/11diary.htm
http://rediff.com/cricket/2002/aug/07field.htm