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Swordfish

Three-line fish: a digit confined to a 3x3 line pattern. expert

686847814859789189145894781487895618956148948891481891914814891489484878158151784714515191791818529417363232523766375219627533694228369674235
Digit 8 in columns 1/5/7 fits only at the marked positions, which line up on rows 2/3/4, one per line, so erase it from everywhere else in rows 2/3/4. (Shown on a plain grid.)

The three-line fish: a digit confined to three columns across three rows locks into that 3x3 line pattern, clearing the digit from the rest of the covered lines.