An Expert 9×9 Puzzle Solved with Forced Chain in 23 Steps

This 9×9 board rates as expert (difficulty score 1159). Solving it from scratch takes 23 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell in column I belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
2. Next, every open cell in column H belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
3. Then, every remaining candidate in this region touches B7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, every remaining candidate in this region touches B8, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
6. From there, every remaining candidate in this region touches D7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
7. Following that, marking A1 would force G2 (the last open cell in region 2), then H4 (the last open cell in region 3), then B9 (the last open cell in region 9), and the chain continues, and then region 8 would be left with no open cell for its marker. So A1 can't be the marker there.
8. First, if B1 were the marker, it would force G2 (the last open cell in region 2), then H4 (the last open cell in region 3), then E3 (the last open cell in row 3), and column F would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, marking D1 would force G2 (the last open cell in region 2), then H4 (the last open cell in region 3), then E9 (the last open cell in region 8), and then row 3 would be left with no open cell for its marker. So D1 can't be the marker there.
10. Then, if E1 were the marker, it would force G9 (the last open cell in column G), and column F would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, that one can't be the marker: at F1 it would force G9 (the last open cell in column G), and column E would be left with no open cell for its marker.
12. Now, every open cell left in this region sits in column G, so its marker has to land there; that clears every other open cell in the column.
13. From there, every open cell in column F belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
14. Following that, place a marker at E9. Column 5 has exactly one open cell left. Placing here clears the rest of row 9, column E, and its region. Placing here also clears its 1 touching neighbour.
15. First, place a marker at F7. Region has exactly one open cell left. Placing here clears the rest of row 7, column F, and its region.
16. Next, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
17. Then, place a marker at C6. Region has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region. Placing here also clears its 2 touching neighbours.
18. After that, place a marker at D4. Region has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region.
19. Now, place a marker at I5. Row 5 has exactly one open cell left. Placing here clears the rest of row 5, column I, and its region.
20. From there, every remaining candidate in this region touches H1 and H2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
21. Following that, place a marker at H3. Region has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region. Placing here also clears its 1 touching neighbour.
22. First, place a marker at G1. Region has exactly one open cell left.
23. Finally, place a marker at B2. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎