The Gra'rak-Gash

The Gra'rak-Gash is a massive pit in the south of Barrova. It is the third of the seven Natural Wonders of Anu. The walls of the pit and the centres of some of the surrounding massive craters are made from a strange rock called Gra not found anywhere else on Anu. It is generally thought that at some point a massive meteorite struck the planet and cleft the Gra'rak-Gash into the land. The remains of ancient flora and fauna in the surrounding wasteland suggest that the continent was once a fully lush expanse of forest and jungle.
It is not known whether the impact of the meteorite, or the Gra it brought with it caused the widespread devastation and aridification of the continent. Due to the radical difference of the local Orcs and Goblins from any other know humanoid on Anu, it is generally believed that their ancestor arrived with the meteorite, either arriving with the actual impact or being changed by the abundance of Gra into what would later become Orcs and Goblins.
The Gra'rak-Gash became a holy site for both the Orcs and their rivals, the Goblins, and both would undertake self sacrifice by jumping into the pit, though neither knew what could be found at the end, if there even was an bottom. As such, both races fiercely competed for the land surrounding the Gra'rak-Gash. The bigger and stronger Orcs would come to hold dominion over the surface area around the pit, establishing many kingdoms and empires along its rim, whereas the goblins established a complex community of city states and tribes in the caves networks around the Gra'rak-Gash, referred to as Gra'Goggalin.
When the Old Empire arrived and conquered the Orc kingdoms and empires around the Gra'rak-Gash they began to investigate the pit. Though they never reached the bottom of the pit, they did discover that Gra seemed to manipulate and corrupt biological tissue, causing mutation and death in prolonged exposure in anyone but the orcs and goblins. Gra was also found to interact with the Winds of Magic, mutating, changing, amplifying and muting it seemingly at random. 
After that revelation, the material became the favourite weapon material for the orc resistances fighting the Imperials. Though Gra exposure sets in only after many minutes or hours of contact, the symbolism of a material hostile to the foreigners led itself well for those who wished to rid themselves of the empire, though they were never truly able to.
Seeking to exploit this special property of Gra, dubbed Decay Magic by the Imperial scholars, the Old Empire began to use Gra in experiments. Most infamously in attempts to create new races according to their design. In these experiments the Old Empire created monstrous races such as Trolls and Gra Monsters as well as many others, most of which were purged by the empire once they were determined to be too aggressive to be useful. Though such experimentation with Gra eventually slowed, this research was later used in the development of the Replicants. More consequentially, imperial mages figured out a process with which they could transform Gra into Mana Crystals a resource that would become vital to the empire's magical prowess and splendour. The empire then took to massive mining operation around the pit, extracting and refining Gra.