Here is the full list of all 28 The 7th Guest: 25th Anniversary Edition achievements.
Discover the "Open House" mode
Complete the entire game
Solve the Knives puzzle
Solve the Skyscraper puzzle
Solve the Knights puzzle
Solve the Stauf's Painting puzzle
Solve the Piano puzzle
Discover the Plant secret passage
Solve the Blocks puzzle
Solve the Flipping Mansion puzzle
Win the Microscope mini-game
Solve the Coins puzzle
Solve the Stepping Stones puzzle
Solve the Cards puzzle
Discover the Bathroom secret passage
Solve the Telescope puzzle
Discover the Fireplace secret passage
Solve the Spiders puzzle
Solve the Crypts puzzle
Solve the Maze puzzle
Solve the Bishops puzzle
Solve the Bed puzzle
Solve the Grate puzzle
Discover the Billiard secret passage
Solve the Queens puzzle
Solve the Cans puzzle
Solve the Heart puzzle
Solve the Cake puzzle