The Deep Alliance was a secret group of dwarfs who inhabited an underground region in the northern trait of the Red Mountains during the Age of Fire. They also called themselves the "Free Northern Dwarfs".
The first of them were survivors of the Punitive expedition on Blacklake and Kark, but in the following years they were joined by others (both from the remains of Wheel of Fire and the Diadem) escaping from the rule of the Dragon Empire and its Hypatian puppets.
At the peak of NiVom and Imfamnia rule, they were among the few ones left to not bent to the Empire, and because of this (and despite their poor state), the Dragon Empire hired the Dragon Tower in destroying the dwarfs (the reason of such quest however could have been actually a secret mission to the band of scouts of Halfmoon, and regarded Seeg's crystal).
However the Copper had joined the Dragon Tower, and he took opportunity to forge an alliance with the dwarfs, thanks his oratorical skills (and promises of food).