It has been nearly Ten thousand years since The Heroes of Old sealed away (Drizzit, Elminster, King Obold, Duedromont, ect, ect) the world of Faerun from the demons and devils. The gods, now able to devote all their attention on the lower planes, ceased most communications.
The paths to such powers as demons possessed became more treacherous then they already had been. The People of Faerun flourished, and while some magics were lost over this time, other were gained. This magic, different from either that of the gods or Wizards, is called Technology.
Unbeknown to the Races, the Demons were breaking the barriers, and recently returned. They have helped a man named Ralthan of Thay to conquer much of the world, lazy and fat from peace.
You are members of the Republics of Icewind, the last free nation in Faerun. Every year the snow melts int he pass, and every year the pass becomes a killing field. when the snows come, ending the Trench warfare that has gone on for the last ten years, demons and Strike forces come through the pass, but it's other end is to heavily defended for a strike from Icewind. they aren't powerful, and are usually half frozen by the time they meet their first enemy. Usually a yeti or One of the tribes, who now patrol the region, meet them first.
Now there is tell of a pass over the mountains, having recently been discovered by the Imperial Legions of Helm, and you are tasked with two things. First, clear the pass. Second, hold it until reinforcements can come to you. Numbers are not on your side, so you must be more cunning.
For the Republics!