The Night Warrior was one of the many strange apparitions used by Malkallam to scare people from visiting Grimsdell Forest. It is a giant warrior, standing over twenty meters tall, but not at all solid, a ghost almost. Malkallam focuses a very bright fire, made from whitestone, through a lens and onto a mist wall to make the Night Warrior appear. Will saw the Night Warrior in book five, The Sorcerer in the North and was completely terrified by it. The Night Warrior makes appearances in books five and six.
In book nine, Halt's Peril, when Will had to pass the night on a barrow-ground on the way to Macindaw, it was hinted that the real Night Warrior haunts the graves.