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 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 in column H belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
2. Next, every remaining candidate in this region touches C4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, marking A1 would force C2 (the last open cell in region 3), then B7 (the last open cell in region 8), then D4 (the last open cell in region 4), and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
4. After that, if B1 were the marker, it would force H2 (the last open cell in row 2), then A6, A7 and A8 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.
5. Now, that one can't be the marker: at C1 it would force A6, A7 and A8 cleared (every open cell left in this region sits in column A, so its marker has to land there), then B7 (the last open cell in region 8), then D8 (the last open cell in region 6), and region 4 would be left with no open cell for its marker.
6. From there, marking D1 would force B2 and B7 cleared (together, two regions have open cells only in columns B and C), then A6, A7 and A8 cleared (every open cell left in this region sits in column A, so its marker has to land there), and then region 8 would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, place a marker at H1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column H, and its region.
8. First, every open cell left in this region sits in row 2, so its marker has to land there; that clears every other open cell in the row.
9. Next, 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.
10. Then, place a marker at B7. Region has exactly one open cell left. Placing here clears the rest of row 7, column B, and its region. Placing here also clears its 2 touching neighbours.
11. After that, together, two regions have open cells only in columns C 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, every remaining candidate in this region touches E3 and F3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
13. From there, marking E2 would force F6 (the last open cell in row 6), then G8 (the last open cell in region 7), then C5 (the last open cell in region 6), and then column D would be left with no open cell for its marker. So E2 can't be the marker there.
14. Following that, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
15. First, place a marker at E6. Row 6 has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region.
16. 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.
17. Then, place a marker at D8. Region has exactly one open cell left. Placing here clears the rest of row 8, column D, and its region.
18. After that, 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.
19. Finally, place a marker at A4. Region has exactly one open cell left.

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