The Arbalessia Fortress arc follows the Millenium Blue arc. The Straw Hat Pirates enter the deeper parts of the Millenium Blue, known as Sand Sea, which is known for its thick, mudlike water one can even walk on (this is found out by Luffy, who for accident, falls out of the ship) Suddenly, a dagger hits the ground right beside Usopps feet. the crew is shocked and anyone gets ready for action. A group of four marine soldiers, wearing knights armour, assault their ship. Franky and Brook defeat the enemys with a combination attack, which includes Franky shooting Brook into the air and the skeleton man performing some ranged rapier attacks to knock the strange attackers into the sky. Shortly after, they reach the great Mangrove Wood, which consumes most of the center parts of Millenium Blue.