A Hard 8×8 Puzzle Solved with Forced Chain in 16 Steps

This 8×8 board rates as hard (difficulty score 1015). Solving it from scratch takes 16 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 column A, so its marker has to land there; that clears every other open cell in the column.
2. Next, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region. Placing here also clears its 2 touching neighbours.
3. Then, 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.
4. After that, together, two regions have open cells only in rows 5 and 6; 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.
5. Now, every remaining candidate in this region touches D2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, every remaining candidate in this region touches G5 and G6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
7. Following that, place a marker at H5. Region has exactly one open cell left. Placing here clears the rest of row 5, column H, and its region.
8. First, place a marker at F6. Region has exactly one open cell left. Placing here also clears its 2 touching neighbours.
9. Next, marking D1 would immediately leave region 1 with no open cell for its marker, so it can't be the marker there.
10. Then, if E1 were the marker, region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, marking G1 would force E2 (the last open cell in region 2), and then row 3 would be left with no open cell for its marker. So G1 can't be the marker there.
12. Now, place a marker at C1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column C, and its region.
13. From there, 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.
14. Following that, place a marker at G2. Region has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region.
15. First, place a marker at E8. Region has exactly one open cell left. Placing here clears the rest of row 8, column E, and its region.
16. Finally, place a marker at A7. Region has exactly one open cell left.

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