The Forsaken War

There's a campaign map up over at the main G+ page for Legacy of the Bieth. On it, you can see that there is an arrow marking where the lands of the Forsaken are.

The Forsaken look like the denizens of Hell as illustrated in Wayne Barlow's Inferno, the results of living for generations in areas ravaged by the now-uncontrolled Bieth magics. They crouch in the remains of Bieth cities, expanding the subterranean tunnels and growing their civilization beneath the sands. They largely keep to the ruined cities, but some have started to expand outwards.

Interior of a Forsaken city.
From Wayne Barlow's Inferno.

About ten years ago, Caliph Abu Qasim decided that he wanted to expand the Empire's borders, secure some trade routes that didn't go right next to a Zone. Taking the results of a few Bani Khalil nomad skirmishes with the Forsaken as justification, he launched an invasion of their lands. It was expected to be a sudden quick strike. Neither the Caliph nor his advisors expected the hordes of Forsaken warriors boiling out from their cities, or the Bieth weaponry and constructs that they brought with them.

Forsaken leader General Hormisdas.
Nathan Rosario's "Portrait of a General"

The fighting went badly for the Empire; whole units were annihilated by the dark fires of Forsaken weaponry. The arrival of the Awlad-i-Dimagh, the new order of Psychic Warriors, was able to stabilize the fighting and even push the Forsaken back across the Muqabla River, where the lines held. Only after the fighting did the Imperium find out about the horrors that the Awlad-i-Dimagh had been concealing...

Imperial troops preparing for war.
Piece at Una The Blade.

It was at this time of stalemate that Caliph Abu Qasim suddenly and tragically passed away - a rapid and sharp onset of acute metal poisoning (a sword through the belly). His daughter Zainab, the new Calipha, apprehended the assassins (personally executing them before anyone could put them to the question), then offered peace to the Forsaken, seeking to end the war and consolidate her new reign. The Empire pledged that it would remain on the eastern side of the Muqabla in return for the Forsaken withdrawing from the ruined city of Qasr al-Malik.

Most Imperial soldiers returned to their homes, but many still haunt Sanctuary as laborers, mercenaries, and burned-out drunks (sometimes filling more than one category).