This is one of the hardest Sudokus (and one of the best Sudokus) I have ever seen, with only 4 given digits. There are some more rules too:
First, normal Sudoku rules apply here.
Both the diagonals also should contain 1 to 9 in some order.
You cannot put the same number in cells which are at a knight's move (in chess) apart.
The center box will be a magic square.
Now comes the real part, can you solve it?
Bonus: Is the solution unique, or can you find any other solutions?
This is not a puzzle of my own devising. However, to avoid spoiling the solve for others I will wait to declare its source until the puzzle is solved. Credit goes to Mark Goodliffe and Simon Anthony for solving this puzzle, and also to Aad Van de Wetering for making such a wonderful puzzle !
Edit: This puzzle got solved here - https://www.youtube.com/watch?v=hAyZ9K2EBF0