Katana is a recurring enemy from the Darius series.
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Katanas appear as big chunks of rock. Being just rocks they don't attack, serving instead as hazards. They move fast from right to the left of the screen, appearing in large quantities. When destroyed, they split up into the smaller Wagatakis.
Unlike other large enemies they'll come in colored variations, though only the Wagatakis will drop power-up orbs.