Psionic tympani, craniomantic amulets, adding the Eighth Corner, qabalistic imprinting of the Mark of Cain, memory engrailment, worship of the Nazca sky-gods who built Sachsahuayman — the question isn’t why drill a hole in your head, but why haven’t you done it yet?
http://www.svonberg.org/2000/12/trepanation/