A Severe 9×9 Puzzle Solved with Forced Chain in 31 Steps

This 9×9 board rates as severe (difficulty score 1489). Solving it from scratch takes 31 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell left in this region sits in row 4, so its marker has to land there; that clears every other open cell in the row.
2. Next, every remaining candidate in this region touches F3 and G3 and F5 and G5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, marking A1 would force C2, D2, G2, and 4 more cleared (together, two regions have open cells only in rows 2 and 3), then H5 (the last open cell in region 5), and then region 2 would be left with no open cell for its marker. So A1 can't be the marker there.
4. After that, if B1 were the marker, it would force A9 (the last open cell in column A), then D2 and D3 cleared (every open cell in column C belongs to the same region, so that region's marker has to be somewhere in this column), then E5, E6, F6, and 4 more cleared (every open cell in column D belongs to the same region, so that region's marker has to be somewhere in this column), and the chain continues, and row 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
5. Now, that one can't be the marker: at C1 it would force A9 (the last open cell in column A), then B3 (the last open cell in column B), then G2, H2 and I2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), and region 4 would be left with no open cell for its marker.
6. From there, marking D1 would force A2, G2, A3, and 1 more cleared (together, two regions have open cells only in rows 2 and 3), then H5 (the last open cell in region 5), then F4 (the last open cell in region 6), and then region 3 would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, that one can't be the marker: at F1 it would force G4 (the last open cell in region 6), and region 5 would be left with no open cell for its marker.
8. First, marking G1 would force F4 (the last open cell in region 6), then H6, H7, H8, and 1 more cleared (every open cell left in this region sits in column H, so its marker has to land there), then I5, I6, I7, and 2 more cleared (every open cell left in this region sits in column I, so its marker has to land there), and then region 8 would be left with no open cell for its marker. So G1 can't be the marker there.
9. Next, if H1 were the marker, region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, that one can't be the marker: at I1 it would force G4, G7 and G8 cleared (together, two regions have open cells only in columns G and H), then F4 (the last open cell in region 6), then E2 (the last open cell in region 3), and the chain continues, and row 9 would be left with no open cell for its marker.
11. After that, place a marker at E1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region. Placing here also clears its 1 touching neighbour.
12. Now, every remaining candidate in this region touches H3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
13. From there, that one can't be the marker: at A2 it would force H5 (the last open cell in region 5), then I3 (the last open cell in region 4), and region 2 would be left with no open cell for its marker.
14. Following that, marking B2 would force H5 (the last open cell in region 5), then I3 (the last open cell in region 4), then F4 (the last open cell in region 6), and the chain continues, and then region 7 would be left with no open cell for its marker. So B2 can't be the marker there.
15. First, if C2 were the marker, it would force H5 (the last open cell in region 5), then I3 (the last open cell in region 4), then F4 (the last open cell in region 6), 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.
16. Next, marking H2 would immediately leave region 5 with no open cell for its marker, so it can't be the marker there.
17. Then, every open cell left in this region sits in column I, so its marker has to land there; that clears every other open cell in the column.
18. After that, together, two regions have open cells only in columns G and H; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
19. Now, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region.
20. From there, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
21. Following that, every open cell left in this region sits in row 9, so its marker has to land there; that clears every other open cell in the row.
22. First, every remaining candidate in this region touches C6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
23. Next, 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.
24. Then, if I2 were the marker, it would force H5 (the last open cell in region 5), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
25. After that, place a marker at I3. Region has exactly one open cell left. Placing here clears the rest of row 3, column I, and its region.
26. Now, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region. Placing here also clears its 2 touching neighbours.
27. From there, place a marker at G2. Region has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region.
28. Following that, place a marker at D7. Region has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region.
29. First, place a marker at B9. Column 2 has exactly one open cell left. Placing here clears the rest of row 9, column B, and its region. Placing here also clears its 1 touching neighbour.
30. Next, place a marker at A6. Region has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
31. Finally, place a marker at H8. Region has exactly one open cell left.

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