Expert Killer Sudoku is where the cages do all the work. A grid usually opens with few or no given digits and the sums alone carry you: single-combination cages first, the 45 rule across boxes and bands, then naked and hidden subsets, wings and fish once the easy arithmetic runs dry.
Every puzzle is graded on the same scale as our classic levels, so "expert" describes the techniques it needs, not how empty it looks. Expect a long solve in note mode with exactly one solution at the end. For the same intensity without arithmetic, try expert classic Sudoku.