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Although it is often mistaken by many residents of Draenor as a natural, crystalline mountain, Oshu'gun / Genedar is actually the dimensional ship which the draenei used to leave their home world of Argus. Originally, the ship probably looked very similar to the dimensional ships which together make up the fortress of Tempest Keep (The Mechanar, The Arcatraz, The Botanica and The Eye).
Arrival on Draenor
Twenty-five thousand years ago the draenei, led by Velen, used Genedar to escape their homeworld of Argus which had been seized by the Burning Legion. Inside the ship was a sizable number of draenei refugees, as well as a few naaru (notably: A'dal, D'ore, K'ara and K'ure).
For reasons unclear, K'ure, the ship's pilot, fell ill, and was forced to crash the Genedar on the surface of Draenor, coming to rest on the Western plains of Nagrand. There is evidence that the ship began breaking apart during its descent, as a large white crystal fell to the ground in Shadowmoon Valley. K'ara was also ejected from the ship above Shadowmoon Valley, at nearly the opposite end of the continent as the bulk of the ship.
The transition from Genedar to Oshu'gun
As the decades passed, Orcs began to revere the "mountain". It became a sacred place and every year the orcs would gather there to celebrate the spring and autumn festival. The shaman would commune with their honored ancestors (who, unknown to the shaman, came to dwell within Oshu'gun to commune with K'ure). The draenei and naaru recognized the Orcs' reverence and retreated, leaving them to their traditions. K'ure, who was injured during the crash-landing, remained in Genedar. Since that time, the "mountain" has mostly only been referred to by it's Orcish name, "Oshu'gun".