Mountain Priests are members of the Miners' guild who perform rituals to placate Mother Earth to permit mining in the earth. Without their rituals, miners would feel the Mother Earth's wrath.
The ritual of the priests is a well-kept secret among them, as it would endanger the monopoly of the Miners' guild on metals and gems.
The ritual itself is performed in the mines. The priests gather around a fire, deeply concentrated, and shout a series of words incomprehensible to outsiders. The ritual is ended with "Umajo Danu".
Only one one outsider, Kossotto of the Equipment Makers' guild, have learned the requirements for the ritual from Tom, in exchange for the Metalmagic Knowledge.