The Free On-line Dictionary of Computing (19 January 2023):

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.


The Jargon File (version 4.4.7, 29 Dec 2003):

deep magic
 n.

    [poss. from C. S. Lewis's Narnia books] An awesomely arcane technique
    central to a program or system, esp. 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 optimization 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. Esp.: found in comments of the form “Deep magic begins here...”.
    Compare voodoo programming.