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

This 9×9 board rates as expert (difficulty score 1323). Solving it from scratch takes 23 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, marking A1 would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then E2, D3, E3, and 3 more cleared (together, two regions have open cells only in rows 2 and 3), then E4 (the last open cell in region 2), and the chain continues, and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
2. Next, marking D1 would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then G3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then F3, H3, I3, and 6 more cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and then region 7 would be left with no open cell for its marker. So D1 can't be the marker there.
3. Then, if E1 were the marker, it would force C2, G2, H2, and 1 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then G3 (the last open cell in region 3), then A4 (the last open cell in region 4), and the chain continues, and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
4. After that, marking G1 would force I2 (the last open cell in region 3), and then region 1 would be left with no open cell for its marker. So G1 can't be the marker there.
5. Now, if H1 were the marker, it would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then C2 and E2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then D3, F3, I3, and 6 more cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, that one can't be the marker: at I1 it would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then C2 and E2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then D3, F3, H3, and 6 more cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and region 7 would be left with no open cell for its marker.
7. Following that, that one can't be the marker: at A2 it would force G3 (the last open cell in region 3), and region 4 would be left with no open cell for its marker.
8. First, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
9. Next, every remaining candidate in this region touches B4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
10. Then, that one can't be the marker: at D2 it would force G3 (the last open cell in region 3), then A4 (the last open cell in region 4), then F1 (the last open cell in region 2), and the chain continues, and region 7 would be left with no open cell for its marker.
11. After that, marking E2 would force G3 (the last open cell in region 3), then A4 (the last open cell in region 4), then C5, H5 and I5 cleared (every open cell left in this region sits in row 5, so its marker has to land there), and the chain continues, and then region 7 would be left with no open cell for its marker. So E2 can't be the marker there.
12. Now, if B1 were the marker, it would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then G3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D3, F3, H3, and 7 more cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
13. From there, that one can't be the marker: at C1 it would force E5, E6, E7, and 2 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then G3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D3, F3, H3, and 6 more cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and region 7 would be left with no open cell for its marker.
14. Following that, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region. Placing here also clears its 2 touching neighbours.
15. First, place a marker at G3. Region has exactly one open cell left. Placing here clears the rest of row 3, column G, and its region. Placing here also clears its 2 touching neighbours.
16. Next, place a marker at A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region.
17. Then, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
18. After that, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
19. Now, place a marker at I6. Region has exactly one open cell left. Placing here clears the rest of row 6, column I, and its region. Placing here also clears its 1 touching neighbour.
20. From there, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region. Placing here also clears its 1 touching neighbour.
21. Following that, place a marker at D5. Region has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region.
22. First, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region.
23. Finally, place a marker at H9. Region has exactly one open cell left.

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