An Expert 11×11 Puzzle Solved with Forced Chain in 24 Steps

This 11×11 board rates as expert (difficulty score 1286). Solving it from scratch takes 24 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, marking A1 would force J2 (the last open cell in region 3), then H3 (the last open cell in region 2), and then region 4 would be left with no open cell for its marker. So A1 can't be the marker there.
2. Next, if B1 were the marker, it would force J2 (the last open cell in region 3), then H3 (the last open cell in region 2), then A4 (the last open cell in region 4), and the chain continues, and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
3. Then, that one can't be the marker: at C1 it would force J2 (the last open cell in region 3), then H3 (the last open cell in region 2), then A4 (the last open cell in region 4), and region 6 would be left with no open cell for its marker.
4. After that, every remaining candidate in this region touches E2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
5. Now, marking D1 would force J2 (the last open cell in region 3), then H3 (the last open cell in region 2), then A4 (the last open cell in region 4), and the chain continues, and then region 10 would be left with no open cell for its marker. So D1 can't be the marker there.
6. From there, that one can't be the marker: at F1 it would force J2 (the last open cell in region 3), then H3 (the last open cell in region 2), then A4 (the last open cell in region 4), and the chain continues, and region 5 would be left with no open cell for its marker.
7. Following that, marking G1 would force F3 (the last open cell in region 1), then J2 (the last open cell in region 3), then A4 (the last open cell in region 4), and the chain continues, and then region 5 would be left with no open cell for its marker. So G1 can't be the marker there.
8. First, if H1 were the marker, it would force I3 (the last open cell in region 2), then F2 (the last open cell in region 1), then A4 (the last open cell in region 4), and the chain continues, and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
9. Next, that one can't be the marker: at I1 it would force F4, F5, F6, and 5 more cleared (every open cell left in this region sits in column F, so its marker has to land there), then A2, B2, C2, and 10 more cleared (together, two regions have open cells only in rows 2 and 3), then A4 (the last open cell in region 4), and the chain continues, and region 5 would be left with no open cell for its marker.
10. Then, every remaining candidate in this region touches K2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
11. After that, marking J1 would force F4, F5, F6, and 5 more cleared (every open cell left in this region sits in column F, so its marker has to land there), then A2, B2, C2, and 8 more cleared (together, two regions have open cells only in rows 2 and 3), then A4 (the last open cell in region 4), and the chain continues, and then region 5 would be left with no open cell for its marker. So J1 can't be the marker there.
12. Now, if K1 were the marker, it would force F4, F5, F6, and 5 more cleared (every open cell left in this region sits in column F, so its marker has to land there), then A2, B2, C2, and 8 more cleared (together, two regions have open cells only in rows 2 and 3), then A4 (the last open cell in region 4), and the chain continues, and column E would end up with no open cell for its marker, which can't happen. So it can't go there.
13. From there, place a marker at J2. Region has exactly one open cell left. Placing here clears the rest of row 2, column J, and its region. Placing here also clears its 2 touching neighbours.
14. Following that, 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. Placing here also clears its 2 touching neighbours.
15. First, place a marker at E1. Region has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region.
16. Next, place a marker at A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region. Placing here also clears its 1 touching neighbour.
17. Then, 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. Placing here also clears its 2 touching neighbours.
18. After that, 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.
19. Now, every open cell left in this region sits in row 6, so its marker has to land there; that clears every other open cell in the row.
20. From there, place a marker at I8. Region has exactly one open cell left. Placing here clears the rest of row 8, column I, and its region.
21. Following that, place a marker at F6. Region has exactly one open cell left. Placing here clears the rest of row 6, column F, and its region.
22. First, place a marker at D10. Region has exactly one open cell left. Placing here clears the rest of row 10, column D, and its region.
23. Next, 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.
24. Finally, place a marker at K11. Region has exactly one open cell left.

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