How to solve this 8×8 logic puzzle

8×8 · severe · 25 steps

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This 8×8 board rates as severe (difficulty score 1263). Solving it from scratch takes 34 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.

First, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
Next, every remaining candidate in this region touches C2 and C4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Then, every remaining candidate in this region touches B3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, every remaining candidate in this region touches D2 and D4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Now, marking A1 would force B4 (the last open cell in region 4), and then region 7 would be left with no open cell for its marker. So A1 can't be the marker there.
From there, if B1 were the marker, it would force A4 (the last open cell in region 4), then H2 (the last open cell in region 3), then C5 (the last open cell in region 7), and region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
Following that, marking D1 would force C3 (the last open cell in region 5), then B5 (the last open cell in region 7), then A2 (the last open cell in region 4), and the chain continues, and then region 2 would be left with no open cell for its marker. So D1 can't be the marker there.
First, if E1 were the marker, region 1 would end up with no open cell for its marker, which can't happen. So it can't go there.
Next, that one can't be the marker: place it at F1, and region 1 would be left with no open cell for its marker.
Then, marking G1 would force E2 (the last open cell in region 1), then C3 (the last open cell in region 5), then A4 (the last open cell in region 4), and then region 7 would be left with no open cell for its marker. So G1 can't be the marker there.
After that, if H1 were the marker, it would force E2 (the last open cell in region 1), then C3 (the last open cell in region 5), then A4 (the last open cell in region 4), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
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. Placing here also clears its 1 touching neighbour.
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. Placing here also clears its 1 touching neighbour.
Following that, every open cell in column E 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.
First, together, two regions have open cells only in rows 2 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.
Next, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
Then, place a marker at B4. Column 2 has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
After that, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region.
Now, every remaining candidate in this region touches F6 and G6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
From there, every open cell left in this region sits in row 7, so its marker has to land there; that clears every other open cell in the row.
Following that, every remaining candidate in this region touches F7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region.
Next, place a marker at F5. Region has exactly one open cell left. Placing here also clears its 1 touching neighbour.
Then, 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.
Finally, place a marker at A6. Region has exactly one open cell left.

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