One Piece Chapter 38, page 19-21: PIRATE FLEET

Suddenly, invisible slashes rip through the air, striking the pirates and the ground. The pirates scream in agony: "UWAAAAAAAHH!!!" and "YAAAH!!!" Luffy is shocked, shouting, "WHAT'S HAPPENING?!!" as the rocks and ground are sliced up around them.

The pirates are mercilessly cut down by the invisible attacks. One of them explains in terror, "IT'S NO USE!!! WHEN HE'S USING HIS 'SILENT STEP' ATTACK, HE CAN'T DISCERN ONE PERSON FROM ANOTHER!! HE'S SO FAST THAT EVEN HE DOESN'T KNOW WHAT HE'S ATTACKING!! HE WON'T STOP THAT 'TIL HE'S TIRED!!!" Nami is horrified, shouting, "WH... WHAT'S GOING ON...?! THOSE PEOPLE ARE GETTING MASSACRED OUT OF NOWHERE!!" Luffy gets slashed on the cheek as the slaughter continues.

Standing in the middle of the chaotic slaughter, Luffy is consumed by rage. He glares into the empty air and screams, "COME OUT, YOU EVIL BUTLER... WHAT THE HELL DO YOU THINK YOUR CREW MEMBERS ARE!??!" Nami looks at Luffy in shock and concern, whispering, "LUFFY...!!?