The Diabolical Cubus Sisters are the fourth boss in The Legend of Zelda: Phantom Hourglass. They are fought on top of the Ghost Ship.
When Link meets one of the Cubus girls (which all four are disguised as normal human beings, descended from the House of Cubus), she will tell him that she, along with her sisters, were taken captive by the Ghost Ship and separated from her siblings and asks for his help in bringing them back together again. Throughout the dungeon, Link rescues four sisters from the ship, eventually reuniting all of them. They agree to thank Link, and bring him to the top of the ship, where they reveal themselves to be ghostly monsters: the Four Cubus Sisters.
Three will shoot laser beams down at Link, while the fourth will send down an energy ball in a manner similar to a game of "Dead Man's Volley". In the fashion of Phantom Ganon, Link must hit the energy ball back at the Cubus Sister. Once the first is gone, the remaining three will hit the energy ball between themselves to disorient Link.
Once only one remains, it will send out three energy balls, but two of them are fake that will harm Link if he hits them. The fake ones look different from the real energy ball, so Link will be able the see the real energy ball easily. Once defeated, a Heart Container is obtained, and Link gains a key to a secret room of the Ghost Ship where Tetra is held and had turned to stone, due to her life force being drained from her by Bellum.