Northern Resistance

The Northern Resistance is the informal, loose collection of freedom fighters, guerrilla forces, radicals and nationalists who oppose the Midaevian Empire and their occupation of the Northern Kingdoms. They were formed from the remnants of various defeated armies that survived the Northern Conquests and have since established themselves in opposition to the Empire and the vassal kings which rule the Northern Kingdoms in their name.
Background
The origins of the Resistance can be traced to the Disaster at Lazuli Falls, where the Alliance army, on its way to reinforce an attack on Tethoris, was trapped by seasonal floods, preventing them from coming to the aid of the Ravernornian rebels, leading to the massacre of that army. A flight of Iron Knights then caught up to the trapped army in the riverlands and killed hundreds of them before the entire force simply dissolved into the wilderness. This is generally accepted as the moment the tide of war turned against the Northern Kingdoms, as with their base of operations secure and supply lines maintained, the Empire was able to move more and more of its legions north, cutting the Northern armies to pieces a bit at a time.  Sensing defeat, many of the Alliance leaders accepted that in order to achieve final victory, they would initially have to endure defeat. And so, even as their armies buckled under the Empire's advance, they started making contingency plans for a long, protracted guerrilla war, setting up supply caches deep in the wilderness, establishing safehouses and moving cash reserves and treasures to underground vaults where they could not be seized. Many of the Northern Kingdoms most influential people, including the Duchess of Tors and Prince Marrel of Jorland would also go underground, acting as regional leaders of the fledgling movement.  With the destruction of the Society of Magic and the assassination of some of its most prominent members, the magic-users of the Northern Kingdoms followed the example of the Resistance, establishing the Conclave, a group of spellcasters from every conceivable discipline who would work towards their own liberation in cooperation with the Resistance. Due to the decentralized nature of the Resistance, which has many cells operating independently of one another in every region of North Erru, it can be difficult for the Conclave to effectively coordinate their activities with the Resistance, leading to moments of chaos and confusion like the Attack on Fort Lurander. 
Strength
Due to the need to keep their operations a secret, and the inherent challenges with recruitment, it is difficult for someone outside of the group to know exactly what the strength of the Resistance is. It is generally accepted that there are dozens if not hundreds of cells scattered across the towns and cities of the Northern Kingdoms, each ranging from half a dozen to a few hundred in size. As well as this, there have been rumours of secret training grounds hidden in the mountains, forests and parts of the Underdark where armies are being drilled and equipped to fight in future uprisings, though as none of these locations have been found this could just as easily be Resistance propaganda, meant to boost the spirits of the people of North Erru.