The main idea with the warband is that tombs and barrows that have been desecrated, had their protective charms stolen or broken or the like, have their inhabitants resting uneasy, and are easily affected by dark magic.
During events such as Geheimnisnacht, when the green moon Morrsleib, made of pure wyrdstone, shines bright, the dark magic in such places can seep into the bones of the dead, and fill them with a parody of life, pulling the souls of the dead back to their bodies, walking the earth again.
There are other undead warbands in Mordheim, the “Undead” warband in the core rulebook led by a Vampire and his minions, another is the “Restless Dead” from the unofficial supplement “Bordertown Burning”, but both these warbands have living creatures in them, and the dead are animated with necromantic spells, I wanted something like a “natural phenomenon” if you could call it that, to have woken the dead.
Anyhow, once I’ve finished the layout in my rules PDF I will upload it and share it, meanwhile it is available on the Mordheim Facebook group.