A Hard 8×8 Puzzle Solved with Forced Chain in 19 Steps

This 8×8 board rates as hard (difficulty score 1108). Solving it from scratch takes 19 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 C3, 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 F8, 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 C7, 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 E2 (the last open cell in region 2), and then region 3 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, if B1 were the marker, it would force D4, D5, D6, and 2 more cleared (every open cell left in this region sits in column D, so its marker has to land there), then C8 (the last open cell in region 7), then A6 and A7 cleared (every open cell left in this region sits in column A, so its marker has to land there), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, that one can't be the marker: at C1 it would force D3 (the last open cell in region 4), and region 7 would be left with no open cell for its marker.
7. Following that, marking D1 would force B2 (the last open cell in region 4), then F3 (the last open cell in region 3), then C8 (the last open cell in region 7), and the chain continues, and then region 1 would be left with no open cell for its marker. So D1 can't be the marker there.
8. First, together, two regions have open cells only in columns E and F; 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, together, two regions have open cells only in rows 1 and 2; 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.
10. Then, place a marker at D3. Region has exactly one open cell left. Placing here clears the rest of row 3, column D, and its region. Placing here also clears its 2 touching neighbours.
11. After that, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region. Placing here also clears its 1 touching neighbour.
12. Now, place a marker at G2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region. Placing here also clears its 1 touching neighbour.
13. From there, place a marker at E1. Region has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region.
14. Following that, 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.
15. 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.
16. Next, every remaining candidate in this region touches B4 and B5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
17. Then, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region. Placing here also clears its 1 touching neighbour.
18. After that, place a marker at A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region.
19. Finally, place a marker at H5. Region has exactly one open cell left.

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