A Severe 9×9 Puzzle Solved with Forced Chain in 35 Steps

This 9×9 board rates as severe (difficulty score 1308). Solving it from scratch takes 35 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, together, two regions have open cells only in columns A 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.
2. Next, 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.
3. Then, every remaining candidate in this region touches C2 and D2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
4. After that, that one can't be the marker: at A2 it would force B6 (the last open cell in region 8), then G8 (the last open cell in row 8), and row 7 would be left with no open cell for its marker.
5. Now, if F2 were the marker, it would force E4 (the last open cell in region 4), then B6 (the last open cell in column B), then A3 (the last open cell in region 1), and the chain continues, and column H would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, marking D1 would force B6 (the last open cell in column B), then G8 (the last open cell in row 8), and then row 7 would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, place a marker at C1. Region has exactly one open cell left. Placing here clears the rest of row 1, column C, and its region.
8. First, every open cell in row 9 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
9. Next, that one can't be the marker: at G2 it would force A8 (the last open cell in row 8), then B4 (the last open cell in region 1), then E6 (the last open cell in row 6), and row 7 would be left with no open cell for its marker.
10. Then, if A3 were the marker, it would force B6 (the last open cell in region 8), then G8 (the last open cell in row 8), and row 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, if D3 were the marker, it would force F4 (the last open cell in region 4), then A5 (the last open cell in region 1), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, marking H2 would force E6 (the last open cell in region 6), then D4 (the last open cell in region 3), and then column B would be left with no open cell for its marker. So H2 can't be the marker there.
13. From there, if I2 were the marker, it would force E6 (the last open cell in region 6), then D4 (the last open cell in region 3), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
14. Following that, place a marker at E2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region. Placing here also clears its 1 touching neighbour.
15. First, every remaining candidate in this region touches G4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
16. Next, that one can't be the marker: at H3 it would force F4 (the last open cell in region 4), then A5 (the last open cell in region 1), and column B would be left with no open cell for its marker.
17. Then, marking I3 would force F4 (the last open cell in region 4), then A5 (the last open cell in region 1), and then column B would be left with no open cell for its marker. So I3 can't be the marker there.
18. After that, place a marker at G3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column G, and its region. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at A8. Row 8 has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
20. From there, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
21. Following that, place a marker at I5. Region has exactly one open cell left. Placing here clears the rest of row 5, column I, and its region. Placing here also clears its 1 touching neighbour.
22. First, place a marker at D6. Region has exactly one open cell left. Placing here clears the rest of row 6, column D, and its region.
23. Next, 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.
24. 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. ∎