A Severe 9×9 Puzzle Solved with Forced Chain in 34 Steps

This 9×9 board rates as severe (difficulty score 1195). Solving it from scratch takes 34 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 1, 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 H2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches B3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, if H1 were the marker, it would force D5 (the last open cell in region 2), then E7 (the last open cell in column E), then F9 (the last open cell in region 9), and the chain continues, and row 8 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 I1 it would force D5 (the last open cell in region 2), then E7 (the last open cell in column E), then F9 (the last open cell in region 9), and the chain continues, and column C would be left with no open cell for its marker.
6. From there, place a marker at G1. Region has exactly one open cell left. Placing here clears the rest of row 1, column G, and its region. Placing here also clears its 1 touching neighbour.
7. Following that, together, two regions have open cells only in columns I 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.
8. First, marking B2 would force D5 (the last open cell in region 2), then A4 (the last open cell in region 5), then C7 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So B2 can't be the marker there.
9. Next, every open cell left in this region sits in column A, so its marker has to land there; that clears every other open cell in the column.
10. Then, together, two regions have open cells only in columns C and B; 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.
11. After that, that one can't be the marker: at D2 it would force A3 (the last open cell in region 1), then C4 (the last open cell in region 5), and region 4 would be left with no open cell for its marker.
12. Now, marking E2 would force A3 (the last open cell in region 1), then C4 (the last open cell in region 5), and then region 4 would be left with no open cell for its marker. So E2 can't be the marker there.
13. From there, every remaining candidate in this region touches E4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
14. Following that, if I2 were the marker, it would force A3 (the last open cell in region 1), then C4 (the last open cell in region 5), and region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
15. First, place a marker at A2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column A, and its region.
16. Next, together, two regions have open cells only in rows 3 and 4; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
17. Then, 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. Placing here also clears its 3 touching neighbours.
18. After that, 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 2 touching neighbours.
19. Now, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region.
20. From there, place a marker at C3. Region has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region.
21. Following that, place a marker at F9. Region has exactly one open cell left. Placing here clears the rest of row 9, column F, and its region.
22. First, place a marker at I8. Region has exactly one open cell left. Placing here clears the rest of row 8, column I, and its region.
23. Finally, place a marker at H4. Region has exactly one open cell left.

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