A charmed skeleton is a human skeleton that has been magically reanimated by a Dark wizard or witch, similarly to Inferi and Zombies. They were found in the Shrieking Shack and attacked anyone they came into contact with by throwing bones at them. Lord Voldemort also used them in the Duel in Little Hangleton.
Guarding the Shrieking Shack
Charmed Skeletons guarded the tunnel from the Whomping Willow to the Shrieking Shack. In 1994, when Harry Potter and Hermione Granger entered the tunnel to save Ron Weasley from Sirius Black, these skeletons attacked them. Harry and Hermione needed to work together to defeat the skeletons, as they only lost an arm if Harry or Hermione attacked them alone.