A Hard 9×9 Puzzle Solved with Forced Chain in 14 Steps

This 9×9 board rates as hard (difficulty score 967). Solving it from scratch takes 14 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Forced chain. Every step below is forced: nothing here is a guess.

1. First, 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.
2. Next, 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. Placing here also clears its 3 touching neighbours.
3. Then, 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.
4. After that, 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. Placing here also clears its 3 touching neighbours.
5. Now, place a marker at E4. Region has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region.
6. From there, place a marker at D1. Region has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
7. Following that, 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.
8. First, place a marker at B9. Region has exactly one open cell left. Placing here clears the rest of row 9, column B, and its region.
9. Next, if F2 were the marker, it would force H8 (the last open cell in region 7), and row 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, if H7 were the marker, row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, if I2 were the marker, it would force F8 (the last open cell in region 8), and row 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region.
13. From there, 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.
14. Finally, place a marker at I8. Region has exactly one open cell left.

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