A master rune is a symbol and word of power that can be visualized in the mind and then spoken to, at least to someone with that ability, incredibly powerful effect. The Book Bound in Pale Leather is filled with them, giving it at least part of its deadly power. These master runes are what the Kencyrath uses to pass from a lost threshold world to another one further down the Chain of Creation.
Jame uses master runes quite frequently—or recklessly after the Book comes into her possession in her first flight from the Master's House, when she uses it to rip a hole through to Rathillien and escape. She utters the rune "BURN" in Dark of the Moon and sets fire to a blizzard;; she uses another one in Seeker's Mask to destroy the Bashtiri Shadow Assassins, blowing away their souls with a wind rune, and then half-chokes another in a berserker flare after Kallystine cuts Jame's face with her razor ring (this later one would have destroyed the senses of all who heard it had Jame not suppressed it; as it was, it blinded and deafened everyone in the vicinity for varying amounts of time and drove the Ardeth Shanir into fits). Later, she shows Ashe the Book Bound in Pale Leather, and the haunt, while horrified that the thing has been carried around like a "change of underwear", automatically assumes, not knowing the extent of Jame's Shanir powers, that Jame cannot use the Book. Jame almost uses another rune near the beginning of To Ride a Rathorn, but teases it apart in her mind so that it does not have to escape and be said.
The pyric rune is a Master Rune used to ignite the pyres of dead Kencyr, which is tradition. Kencyr priests all appear to know the rune, as does Kindrie from his time in the Priests' College at Wilden.