A Hard 8×8 Puzzle Solved with Forced Chain in 21 Steps

This 8×8 board rates as hard (difficulty score 1114). Solving it from scratch takes 21 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, 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 E, so its marker has to land there; that clears every other open cell in the column.
2. Next, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
3. Then, every remaining candidate in this region touches G4 and E5 and G5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
4. After that, every remaining candidate in this region touches B6, 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 D6 and D7, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
6. From there, marking A1 would force B5 (the last open cell in region 6), and then region 4 would be left with no open cell for its marker. So A1 can't be the marker there.
7. Following that, if B1 were the marker, it would force H2 cleared (every open cell left in this region sits in row 2, so its marker has to land there), then A4 and A8 cleared (every open cell left in this region sits in column A, so its marker has to land there), then C4, C5 and C6 cleared (every open cell left in this region sits in column C, so its marker has to land there), and the chain continues, and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
8. First, that one can't be the marker: at C1 it would force G2 (the last open cell in region 2), and row 3 would be left with no open cell for its marker.
9. Next, 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.
10. Then, marking D1 would force H2, H3, H4, and 4 more cleared (every open cell in column G belongs to the same region, so that region's marker has to be somewhere in this column), and then column H would be left with no open cell for its marker. So D1 can't be the marker there.
11. After that, every remaining candidate in this region touches H2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
12. Now, every open cell in row 2 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.
13. From there, every open cell in row 3 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.
14. Following that, place a marker at D5. Column 4 has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 1 touching neighbour.
15. First, place a marker at F4. Region has exactly one open cell left. Placing here also clears its 1 touching neighbour.
16. Next, place a marker at H3. Region has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
17. Then, place a marker at G1. Region has exactly one open cell left.
18. After that, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region.
19. Now, 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.
20. From there, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region.
21. Finally, place a marker at C8. Region has exactly one open cell left.

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