Logic Masters Deutschland e.V.

Rules:
Quattroquadri: Place the digits 1 to 9 once each in the 3x3 boxes. Digits may not repeat in the same row, column or box.

Dutch Whispers: Adjacent digits on an orange line must differ by at least 4.
German Whispers: Adjacent digits on a light green line must differ by at least 5.
Greater/Less-Than: A small thin pointy arrow between two cells points to the smaller digit.
Modular Line: Digits along a teal/dark green Modular Line must yield a different remainder when they get divided by 3.
Negative Diagonal: Digits along the negative diagonal cannot repeat.
Numbered Rooms: The digit closest to the clue gives the position in the row/column of the digit matching the clue, counting from the clue's side of the grid.
Palindrome: Digits along a gray Palindrome line read the same forwards and backwards.
Quadruples: Digits in a white circle must appear in the four surrounding cells.
Sandwich: Numbers outside of the grid indicate the sum of the digits "sandwiched" between the highest and lowest digits in the corresponding row or column.
Skyscraper: Outside digits indicate the number of seen "buildings", where heights of buildings are indicated by the digits in the grid and taller buildings obscure sight of shorter buildings.
V: Digits separated by a thick V must sum to 5.
X: Digits separated by an X must sum to 10.
X-Sum: Outside clues indicate the sum of the first X digits in that row or column, where X is the first digit seen from that direction.

Puzzle Link: You can solve the puzzle in SudokuPad.