Something in the Twilight lands is stirring.
When the Healing of the World happened, the parallel worlds of Faerie and Shadow were healed as well. But in the case of Shadow, a place that was always wrong, somehow always broken, the Healing caused massive scars throughout the newly-unified land of the restless dead, stirring awake things long forgotten. These scars manifest as weak points in the barrier between the world of the living and the realm of the dead.
The most powerful necromancers of Erandor already dwelled within the area known as the Twilight Lands and when they sensed the thinning of the barrier to the realm Shadow, they tore them open with the aim of enslaving spirits that tumbled through.
They got more than they bargained for.
Nature of the Shadow Realm
The Shadow Realm is not home to all spirits of the dead. In truth, many of the souls of Erandor reincarnate into other living things. Others go to join their gods in higher realms for a time. But some lost souls never make it to any kind of afterlife or incarnation. Perhaps they were too confused at the time of their deaths, or unaware they had died, reviled by their own gods, or any number of other reasons. The one thing these souls have in common is that they wander the Shadow Realm restlessly.
This otherworldly dimension resembles Erandor — in all its incarnations and eras. You can find traces of ancient empires, civilizations long dead, still reflected in part here right along side the newest vision of Erandor.
This makes Shadow seem endlessly vast, yet travel is more about intent than distance. If your will is strong enough, you will find your way to that which you seek. But if your will is weak, you may wander forever.