A Hard 10×10 Puzzle Solved with Forced Chain in 21 Steps

This 10×10 board rates as hard (difficulty score 1032). Solving it from scratch takes 21 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, 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.
2. Next, place a marker at E3. Region has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region. Placing here also clears its 3 touching neighbours.
3. Then, place a marker at I4. Region has exactly one open cell left. Placing here clears the rest of row 4, column I, and its region. Placing here also clears its 2 touching neighbours.
4. After that, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
5. Now, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
6. From there, every open cell in column A 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.
7. Following that, every open cell in column B 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.
8. First, every remaining candidate in this region touches G1, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
9. Next, every remaining candidate in this region touches D5 and B6 and D6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
10. Then, place a marker at D7. Region has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region. Placing here also clears its 1 touching neighbour.
11. After that, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region.
12. Now, every remaining candidate in this region touches A8 and A9, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
13. From there, if H1 were the marker, row 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
14. Following that, every open cell left in this region sits in column J, so its marker has to land there; that clears every other open cell in the column.
15. First, if A6 were the marker, it would force H9 (the last open cell in region 6), and row 10 would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, place a marker at A10. Region has exactly one open cell left. Placing here clears the rest of row 10, column A, and its region. Placing here also clears its 1 touching neighbour.
17. Then, place a marker at B8. Region has exactly one open cell left. Placing here clears the rest of row 8, column B, and its region.
18. After that, place a marker at G9. Region has exactly one open cell left. Placing here clears the rest of row 9, column G, and its region.
19. Now, 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.
20. From there, place a marker at J2. Region has exactly one open cell left.
21. Finally, place a marker at H6. Region has exactly one open cell left.

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