Arlong is a fictional anthropomorphic shark-man from the anime and manga known as One Piece.
Considered to be one of the most powerful and ruthless pirates alive. Arlong is a prideful Merman Supremacist that believes all humans were created to serve Mermen.
Mermen in One Piece are a race of anthromorphic fish people with both lungs and gills. They're said to be stronger then humans above water, and ten times stronger and faster than humans under water.
Arlong's domain is a large island and several small villages called Arlong Park. He intends someday to take over the ocean and create a great Merman empire.
Wikipedia contains spoilers
When Arlong first arrived on the island that would someday become Arlong Park, he killed Nami's adoptive mother and kidnapped Nami. Later he forced her to become a part of her crew as a map maker, and told her that if she brought him a million Belli (this setting's cash) he would free her village and everyone in it, including her sister.
Years later, when Nami was about to fullfill her end of the bargain using money she had stolen from Luffy D. Monkey, Arlong arranged to have some corrupt Marines confiscate the money from her so she would be forever under his control.
Luffy, who followed Nami after he was robbed by her, saw how distraught she was and without questions went to fight Arlong. What ensued was a long and arduous battle between Luffy, Arlong, and the elites of their crews.
By the end it's left with only Luffy and Arlong standing and Luffy is taking a beating. It doesn't look very good for Luffy until the battle moves into the top of Arlong's tower where Nami slavishly made her maps for Arlong.
After feeling the turmoil and desperation that had seeped into the room over the years, Luffy became enraged and the tables turned dramatically.
With one massive blow, Luffy snapped Arlong's neck and demolished his entire tower. Only Luffy walked out of the rubble alive, so for all intents and purposes it appears Arlong is dead.
