This is a new tradition started by The Royal Queen. Whenever new kings are going to be called forth for the English section of New Kingdom, they are said to be birth by The Mountain of the Kings.
The kings are called forth, and spend a brief space of time within the mountain itself. How they enter the mountain is unclear, it’s a bit of a mystery, but once they leave, if they survive their stay, they are then cristened into the post of being the New Kings, for the English section of New Kingdom.
The Royal Queen's Blog 