Source : Free On-Line Dictionary of Computing

deep magic
     
        [possibly from C. S. Lewis's "Narnia" books] An awesomely
        arcane technique central to a program or system, especially
        one neither generally published nor available to hackers at
        large (compare {black art}); one that could only have been
        composed by a true {wizard}.  Compiler optimisation techniques
        and many aspects of {OS} design used to be {deep magic}; many
        techniques in cryptography, signal processing, graphics, and
        AI still are.  Compare {heavy wizardry}.  Especially found in
        comments of the form "Deep magic begins here.".  Compare
        {voodoo programming}.