Logic Masters Deutschland e.V.

Normal Sudoku and Normal Killer rules apply. Redgrey squares are even.
Digits 1 to 6 on the outer ring can not see an identical digit on the outer ring diagonally, but they must see exactly one identical digit within the white box on a diagonal, digits with 2 diagonals can still only see 1 identical digit.
Digits 7 and 8 must see an identical digit on the outer ring, which can later be obstructed by an identical digit in the box.
As for the 9's, there's a diagonal with 3 of them.

Johnny Cash Puzzle

Square of fire just didn't sound right to me.