Laerlee cannot stay with the PCs; she hopes to gather enough strength to destroy the wraiths before they reach the Dales. If she stands with the PCs and they fall or are by-passed by some wraiths, no warning will come to the Dales before cold death arrives.
If the PCs tarry or turn back, the DM should let them see a dark cloud moving over the land. If it is stared at, the viewer can dimly see armored, silent warriors, striding tirelessly forward with drawn swords and cruel eyes of white flame.
Each wraith carries with it a sword upon which continual darkness has been cast. The spell effect is very localized (barely covering the attendant wraith), serving to protect the undead things rather than act as a weapon in combat. If these wraiths are exposed to full sunlight, they will recoil for a few rounds, and cannot drain or chill by their attacks. They can still do physical damage with weapons (the swords), but can in turn be struck by all sorts of weapons. There are 32 wraiths, and each has 36 hp.