A Severe 8×8 Puzzle Solved with Forced Chain in 33 Steps

This 8×8 board rates as severe (difficulty score 1581). Solving it from scratch takes 33 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 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.
2. Next, 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.
3. Then, every remaining candidate in this region touches D6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, marking A1 would force B6 (the last open cell in region 8), then F5, G5 and H5 cleared (every open cell left in this region sits in row 5, so its marker has to land there), then C8, D8, E8, and 3 more cleared (every open cell in row 7 belongs to the same region, so that region's marker has to be somewhere in this row), and then row 8 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, 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.
6. From there, if B1 were the marker, it would force A5, A6, A7, and 1 more cleared (every open cell left in this region sits in column A, so its marker has to land there), and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
7. Following that, that one can't be the marker: at C1 it would force A5, A6, A7, and 1 more cleared (every open cell left in this region sits in column A, so its marker has to land there), then B6 (the last open cell in region 8), then D4 (the last open cell in region 4), and region 6 would be left with no open cell for its marker.
8. First, if E1 were the marker, it would force D3 (the last open cell in region 2), then C8 cleared (every open cell left in this region sits in column C, so its marker has to land there), then A4 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, 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, that one can't be the marker: at A2 it would force B6 (the last open cell in region 8), then F5, G5 and H5 cleared (every open cell left in this region sits in row 5, so its marker has to land there), then C8, D8, E8, and 3 more cleared (every open cell in row 7 belongs to the same region, so that region's marker has to be somewhere in this row), and row 8 would be left with no open cell for its marker.
10. Then, if C2 were the marker, it would force E5, E6, E7, and 1 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then D5 (the last open cell in region 6), then E3 (the last open cell in region 2), and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, together, two regions have open cells only in columns B and A; 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.
12. Now, every remaining candidate in this region touches D3, 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 D2 it would force C4 (the last open cell in region 4), then A3 (the last open cell in region 1), then B6 (the last open cell in region 8), and the chain continues, and region 5 would be left with no open cell for its marker.
14. Following that, that one can't be the marker: at F1 it would force B2 (the last open cell in row 2), then E5, E6, E7, and 1 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then E4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and region 6 would be left with no open cell for its marker.
15. First, marking E2 would force B6 (the last open cell in column B), then D5 (the last open cell in region 6), then C3 (the last open cell in region 4), and the chain continues, and then region 5 would be left with no open cell for its marker. So E2 can't be the marker there.
16. Next, marking G1 would force B2 (the last open cell in row 2), then E5, E6, E7, and 1 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then E4 and F4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and then region 6 would be left with no open cell for its marker. So G1 can't be the marker there.
17. Then, if H1 were the marker, it would force B2 (the last open cell in row 2), then E5, E6, E7, and 1 more cleared (every open cell left in this region sits in column E, so its marker has to land there), then E4 and F4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), and the chain continues, and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
18. After that, place a marker at D1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
19. 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.
20. From there, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
21. Following that, every remaining candidate in this region touches G3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
22. First, every remaining candidate in this region touches F5 and F6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
23. Next, if F2 were the marker, it would force G5 (the last open cell in region 5), then E6 (the last open cell in region 6), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
24. Then, that one can't be the marker: at G2 it would force F4 (the last open cell in region 5), then A3 (the last open cell in region 1), and column C would be left with no open cell for its marker.
25. After that, every open cell left in this region sits in column H, so its marker has to land there; that clears every other open cell in the column.
26. Now, marking H2 would force B6 (the last open cell in column B), then E5 (the last open cell in region 6), then F3 (the last open cell in region 5), and the chain continues, and then column C would be left with no open cell for its marker. So H2 can't be the marker there.
27. From there, place a marker at B2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region. Placing here also clears its 1 touching neighbour.
28. Following that, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region.
29. First, 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.
30. Next, place a marker at G5. Region has exactly one open cell left. Placing here clears the rest of row 5, column G, and its region.
31. Then, place a marker at E6. Region has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region. Placing here also clears its 1 touching neighbour.
32. After that, place a marker at F8. Region has exactly one open cell left. Placing here clears the rest of row 8, column F, and its region.
33. Finally, place a marker at A7. Region has exactly one open cell left.

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