The subject of this article is a custom that has been mentioned "in-universe" in a canon source. The HDM Wiki is written from the perspective that all information presented in canon is true (e.g. Lyra's world really exists); as such, details contained in this article may differ from real world facts.
Trepanning was a thousands-year-old process in which a hole was drilled into the skull of a living human. Tartars and Skraelings were said to practice trepanning.
In the Tunguska Campaign, Lee Scoresby captured five Tartars alive and three had holes in their skulls. He told Lyra Silvertongue that it was a great privilege among Tartars. The process was believed to let the gods talk to those with holes in their heads.