Vander Decken IX is a secondary antagonist in the Fishman Island Arc of One Piece.
He is the nine generations below Vander Decken, a Pirate who's name was well known and brought fear to Brook upon hearing of him. Vander Decken IX had been stalking the mermaid princess, Shirahoshi, since she was six years old. She was locked away for 10 years for protection while Decken threw knives and axes at the door constantly.
During Fishman Island Arc
Decken was significant in the arc because he used his devil fruit power to carry a gigantic ship called the Noah up above the island and was going to drop it on it. Shirahoshi ran away and when he tried to follow her, Hody Jones came up and stopped him.