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

This 8×8 board rates as sharp (difficulty score 1029). 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 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, place a marker at D2. Region has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region. Placing here also clears its 3 touching neighbours.
3. Then, 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.
4. After that, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
5. Now, every remaining candidate in this region touches B4, 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 A5 and C5 and C6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
7. Following that, together, two regions have open cells only in rows 3 and 4; 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.
8. First, 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.
9. Next, marking G1 would force C4 (the last open cell in region 4), and then row 5 would be left with no open cell for its marker. So G1 can't be the marker there.
10. Then, if G3 were the marker, it would force A4 (the last open cell in region 1), and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
11. After that, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region. Placing here also clears its 1 touching neighbour.
12. Now, place a marker at A3. Region has exactly one open cell left.
13. From there, 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.
14. Following that, 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.
15. First, every open cell left in this region sits in row 8, so its marker has to land there; that clears every other open cell in the row.
16. Next, every remaining candidate in this region touches E7 and F7, 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 H7. Region has exactly one open cell left. Placing here clears the rest of row 7, column H, and its region.
18. After that, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
19. Finally, place a marker at E8. Region has exactly one open cell left.

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