A full moon

In Tarker's Mills, Maine a strange occurrence is happening every month when the moon is full. Everytime someone dies in the most vicious way. Blood is shed every month and none is spared: man, woman nor child. Initially, it is thought an animal is the killer but there's something more sinister going on.

On the fourth of July, when ten year old Marty Coslaw is out in his wheelchair with some fireworks he's gotten from his uncle something is lurking behind him. But Marty wounds the beast with one of the arrows. Shortly after it's clear that the beast he saw was actually a stand up member of the town who has a dark secret and now Marty knows about his secret as well and this endangers his and his family's life. Will they escape the werewolf?
Stephen King admits in his afterword how he took some liberties with the lunar cycle so it fits perfectly in his story so that each full moon collides with a holiday. This book started out originally as a calendar where every month a part of the story would be published accompanied with the amazing drawings by Bernie Wrightson (who already did the artwork for the Creepshow graphic novel and would later add art for The Stand and The Wolves of the Calla, the fifth instalment in the 'The Dark Tower' series by King.

This is a novella, partly graphic novel, but it still is one of the most brutal King stories that haunts you long after you read it.
In 1985 Dan Attias adapted this story into a feature movie with Corey Haim as Marty Coslaw and Gary Bussey as his uncle Red, but the title was changed to 'Silver Bullet' and the story was expanded.