A Hard 9×9 Puzzle Solved with Forced Chain in 17 Steps

This 9×9 board rates as hard (difficulty score 1021). Solving it from scratch takes 17 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, 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.
2. Next, every remaining candidate in this region touches B6 and B7, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches C8, 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 B1 were the marker, it would force F2 (the last open cell in region 2), then C9 (the last open cell in region 9), then D6 (the last open cell in region 6), and region 3 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 F2 (the last open cell in region 2), then B4 and B5 cleared (every open cell left in this region sits in column B, so its marker has to land there), then D6 (the last open cell in region 6), and region 3 would be left with no open cell for its marker.
6. From there, marking D1 would force F2 (the last open cell in region 2), then I3, I4, G5, and 6 more cleared (every open cell in column H belongs to the same region, so that region's marker has to be somewhere in this column), and then column I would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, every open cell in row 1 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.
8. First, together, two regions have open cells only in columns B and C; 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.
9. Next, 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. Placing here also clears its 2 touching neighbours.
10. Then, place a marker at E2. Region has exactly one open cell left. Placing here clears the rest of row 2, column E, and its region. Placing here also clears its 2 touching neighbours.
11. After that, place a marker at B3. Region has exactly one open cell left. Placing here clears the rest of row 3, column B, and its region.
12. Now, place a marker at A7. Region has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region.
13. From there, place a marker at C9. Region has exactly one open cell left. Placing here clears the rest of row 9, column C, and its region.
14. Following 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.
15. First, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region. Placing here also clears its 1 touching neighbour.
16. Next, 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.
17. Finally, place a marker at H1. Region has exactly one open cell left.

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