In the Pre-Unified Age, thousands of years before Common Unification, Diarmuid was the king of Copperbreach, a peripheral kingdom in the Snoutridge. His reputation as a warrior and tactician grew to legendary status during the War Of The Dark Dwarves, and he was named High King of the Kingdom of the Twelve Mountains. He ruled from Copperbreach for years, but its remote location from the other High Clans created problems with communication.
This especially proved to be a problem for recording the Age of Giants Timeline; it was spoken that Diarmuid was killed in an assault on Copperbreach by Giants, but little detail was actually known about the event. All that was recorded was that, centuries after his death, his house line Brasshand eventually moved from Copperbreach to the new capital of Brazenthrone.