Once, the various races of the world lived on the surface, a place lush with beautiful sights, towering trees and vast oceans, with a blue sun illuminating the world.
A great calamity fell upon the world. The sky darkened, the grass turned from it's natural colour to a crimson, as did the sky.
Something had corrupted the very sun. It now shone with a red light, and all that it touched became corrupted, malformed. Only sentient beings were saved, initially.
Blood began to shed from the sky, and where it dropped, monsters were formed. Malignant beasts full of only malevolence and hatred. Those who fought back were corrupted, their souls consumed. The Elaudir, the primary sentient race, fled underground. Only they found it had changed beyond belief.
Their world was impossibly deep, with chambers connected to chambers, each going deeper and deeper. There seemed to be no end.
Until one man found something, in the deepest chamber of the world. He described it only as a mountain of whirling energy, barely contained with rock.
There were those who doubted him, but most craved the hope something like that could bring. Hope that something of such power could save their world.
Thus, the Mountaineers were born. Droves of people descended into the dark, many not to return.
Some made it far, others died immediately to the perils of Mountaineering.
One group came together to establish a guild, one that would have routes in any nations that formed under the surface, with the sole goal of finding the Mountain.
Nothing is impossible in the abyss.