Here is a (very hard) sample puzzle from the Killer Sudoku puzzle pack by Serkan Yürekli, which was just added to our e-store.
Theme: Time Killer
Author/Opus: This puzzle comes from our managing editor Serkan Yürekli as part of his new e-book.
Rules: Standard Sudoku rules. Also, the sum of the digits in each cage must equal the value given in the upper-left corner of that cage. Digits cannot repeat inside a cage.
Difficulty: 4.5 stars
Time Standards (highlight to view): Grandmaster = 11:00, Master = 24:00, Expert = 48:00