A Hard 8×8 Puzzle Solved with Forced Chain in 18 Steps

This 8×8 board rates as hard (difficulty score 1029). Solving it from scratch takes 18 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, together, two regions have open cells only in rows 2 and 3; 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.
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, every remaining candidate in this region touches B2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, every remaining candidate in this region touches G7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, marking A1 would force B3 (the last open cell in region 3), then C5 (the last open cell in region 6), then E4 (the last open cell in region 4), and the chain continues, and then region 7 would be left with no open cell for its marker. So A1 can't be the marker there.
7. Following that, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
8. First, if B1 were the marker, it would force A3 (the last open cell in region 3), then C5 (the last open cell in region 6), then E4 (the last open cell in region 4), and the chain continues, and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, 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. Placing here also clears its 1 touching neighbour.
10. Then, 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.
11. After that, together, two regions have open cells only in columns E and D; 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, 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. Placing here also clears its 2 touching neighbours.
13. From there, place a marker at G5. Column 7 has exactly one open cell left. Placing here clears the rest of row 5, column G, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, place a marker at H8. Region has exactly one open cell left. Placing here clears the rest of row 8, column H, and its region.
15. First, 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.
16. Next, 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.
17. 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.
18. Finally, place a marker at D4. Region has exactly one open cell left.

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