variant sudoku is a kind of sudoku that uses additional sets of rules to help you find the solution to the puzzle. for example, a pretty common additional ruleset is "killer cages" (from a site that generates killer sudoku algorithmically):
regular sudoku rules still apply (the digits 1-9 must be placed exactly once in each row, column, and 3x3 box), but now there are "cages", boxy shapes drawn with dotted lines. digits can't repeat in cages at all, and they must sum to the digit in the top left corner. it might surprise you to discover that the above puzzle is pretty easily solvable, even with no given digits, just because of these rules!
there's other common rulesets like "renban lines" or "anti-knight" or "dutch flatmates" or "XV" (which i wont get into now). but people can also make up their own new rulesets. for instance!
the Rat Run series by Marty Sears (marty_sears) is probably the most iconic series of variant sudoku, and more accessible than the other puzzles i'll talk about. each puzzle has you drawing a path to guide a rat named Finkz (i believe named for a puzzle setter who goes by Ratfinkz) through a maze to find a cupcake. the path has to follow certain rules—e.g. in the above puzzle, any two adjacent cells along the path must sum to a prime number, so Finkz can move from a 2 to a 3, but not from a 2 to a 4—so solving the maze and solving the sudoku are the same thing, and doing one also helps you do the other. i mentioned a cloning-based intrigue plot, but you will have to discover that for yourself >:)
speaking of discovering things for yourself:
One Fine Day by Michael Lefkowitz a.k.a. juggler, released yesterday, is the point-and-click adventure sudoku. on the logic-masters page for the puzzle, the only given rules are "Sudoku rules always apply. The rest is up to you to discover."
you may notice that the green rules text box to the right doesn't contain rules, only emojis. those emojis can be clicked to navigate to other puzzles, and only by finding those puzzles can you learn the rules necessary to eventually complete the main grid. worth noting that this requires a pretty robust knowledge of sudoku variants (like, if you don't know what a "kropki dot" is, youre probably gonna struggle) so that you can use that knowledge to intuit what certain elements in the puzzles mean, since there is never any text explaining what the rules are (pretty nonstandard for a sudoku!)
and finally, perhaps most ambitious of all:
Sudokuvania: Digits of Despair; its sequel, Sudokuvania 2: Lineage of Logic (both by SkepticalMario); and Super Sudokoid: Final Transmission (by The M, inspired by the first Sudokuvania) are all honest-to-god sudoku metroidvanias. they make use of the "Dynamic Fog" mechanic in the wonderful sudokupad app, which covers the grid in a gray "fog" that gets cleared in specific places as you solve the puzzle. this allows the solver to "discover" new abilities by uncovering fog from text boxes and other parts of the grid as they progress. the puzzle has "items", "collectibles", even boss fights in the form of other puzzles that can be accessed once you discover the cell a boss is in. they're monster puzzles (the sudokupad timer feature tells me i spent over 300 hours in the first sudokuvania) and pretty difficult, so this is also definitely not for beginners.
if you're interested in trying variant sudoku for yourself, i recommend the youtube channel Cracking the Cryptic—watching other people solve puzzles can be a great way to learn how to solve them yourself!—and especially their series of videos on GAS or Genuinely Approachable Sudoku. if you don't want to watch the videos, you can always just click through to the puzzles in the description, anything they cover on that channel is generally pretty good quality.
once you get familiar with variant sudoku, you can check out logic-masters.de and see what puzzles catch your eye (you may have to change the language by clicking the flag in the top right corner). the number of stars indicates the average difficulty as rated by people who've solved it and the percentage indicates the average quality score (if the stars are grayed out, that's the difficulty estimated by the puzzle's setter).
obviously not every puzzle is at the same impressive level of ambition as the ones listed above, but there's still a lot of cool and clever and beautiful puzzles, and if nothing else it's a lot of fun and helps stretch your brain in new ways. and if you have any questions please send an ask my way! can't guarantee i'll know everything but i might have a good idea of where to direct you.
