For those who like to live dangerously and push beyond their mental comfort zone, steel yourself for The Times' toughest Killer collection yet.

Prepare for the extreme mental challenge of 100 Deadly-level Killer puzzles, including, for the first time ever, 20 Extra Deadly, to really test your limits: thjs is the only place you'll find such a torturous collection in a single book.

The Times Ultimate Killer Su Doku is not for the fainthearted. The puzzles use the same 9x9 grid as Su Doku, and have an added mathematical challenge. The aim is not only to complete every row, column and cube so that it contains the digits 1-9, it is also necessary to ensure that the outlined cubes add up to the same number as well.

But unlike with previous books, there is no chance to ease yourself in with Easy, or even Tough puzzles: this book is the ultimate Killer Su Doku - completely deadly.