There is an episode in Guy Gavriel Kay's 1990 novel Tigana, appearing in an answer I previously posted here, which matches this.
The character Baerd comes upon a village of people ("Night Walkers") who fight against an evil spreading across hills and fields. The fight happens in a dreamland once a year at the spring solstice. In the dream, the evil manifests as "The Others", evil creatures (which appear different to each person that sees them) spreading across the landscape. Lost ground in the dream-world translates to the real ground's loss of ability to grow crops. A death in the dream-world results in the real person's quick sickening and death.
The villagers have always been on the defensive and continually losing ground.
In the story, Baerd happens upon the Night Walkers on the precise night of the yearly battle, and joins in. His efforts turn the tide and drive the Others and their shadowy leader away.
Although this episode is seemingly disconnected from the main story, several things are implied in Baerd's conversations:
There seems to be a spiritual connection between the spreading evil in the dream and the physical evil happening in the Palm: Its division between Alberico and Brandin, and especially Brandin's ongoing genocide in Tigana.
The Night Walkers' driving back the spreading evil with the help of Baerd was necessary for the eventual outcome of the book.
the Night Walkers have an actual role in the final battle.