Before the mine's collapse, Grey Wardens entered the Deep Roads here and found an unnamed thaig. They believed that this thaig was home to one of the six archdemons. As they got deeper into the earth, the more dangerous it became, but the increased calling from the archdemon made them push forward, despite losing men to cave-ins. The Wardens, however, lost too many before they managed to get to the archdemon and kill it before it wakes up and starts a Blight, and they had to turn back.
It is unknown if it was one of the five archdemons that have awakened by 9:41 Dragon, or if it was Lusacan, one of the two remaining Old Gods not to be awakened, as it couldn't have been Razikale, as Razikale is the only known female Old God.
The Inquisitor can find a Warden logbook describing an unknown Grey Warden's experience at the mine, and the regret over not being able to reach the archdemon.
By 9:41 Dragon, the mine is completely collapsed and covered in lava.