Swordfish is X-Wing stretched to three rows and three columns, with the same relaxation naked triples got over naked pairs: no row needs exactly three candidate cells. Two or three is fine, as long as three rows’ worth of candidate cells for one digit fall into exactly three columns total.

The pattern

Pick a digit and look for three rows where it has two or three remaining candidate cells each. If the union of all the columns those cells sit in is exactly three columns — not four, not two — you have a swordfish. The digit is confined to those three rows and three columns together, so every other cell in those three columns can have it eliminated.

This is genuinely harder to spot than X-Wing, because the rows don’t each need to look the same — one might have the digit in columns 2 and 5, another in columns 5 and 8, a third in columns 2 and 8. No single row shows all three columns, but together they never go outside them.

Worked example

7
1
3
9
2
6
5
8
4
8
59
569
3
1
4
79
2
67
4
2
69
78
78
5
39
1
36
5
34
48
2
9
78
1
6
37
6
39
89
78
5
1
4
37
2
2
7
1
6
4
3
8
9
5
1
6
2
4
378
78
37
5
9
9
45
45
1
37
2
6
37
8
3
8
7
5
6
9
2
4
1

7 forms a Swordfish across rows 3, 5, 8, confined to columns 4, 5, 8 — clearing 7 from the rest of those columns.

How to find one quickly

Go digit by digit and list, for each row, which columns could still hold it when the count is two or three. Then look for any three rows whose column lists only ever mention three columns combined. It helps to do this on paper or in notes mode rather than trying to hold it all in your head — tracking three rows at once is the whole difficulty here.

This only comes up once X-Wing has already been tried and failed. If swordfish doesn’t find anything either, XY-Wing is a completely different approach worth trying before assuming the puzzle needs something beyond what’s on this site.

Puzzles at Evil are the first level that may need this.