Write a plot outline for a mystery novel in which a group of close-knit friends must solve a gruesome murder that takes place inside a grand, isolated mansion. The story should feature multiple suspects, clues and red herrings, and plenty of twists and turns to keep the reader guessing until the very end. The style should be atmospheric and suspenseful, with vivid descriptions of the mansion, its surroundings, and the characters' emotions and motivations.
This prompt doesn't seem malicious.