The Massacre is owned by the Kira pirates, the crew of the Shichibukai Lixis D. Korosume
It is a rather large ship that seems very grim and quiet. It is almost impossible to leave the ship alive. The entire ship is booby trapped, or dangerous on its own. The figurehead is the grim reaper, but if you search carefully enough, you would see that there are hidden plasma rays inside the eyes. The ship is made of the roots of the Adam Tree, which is even better than its bark.
There are different rooms which have different purposes. For example one of them might be the weapons storage, or the trap room. Or even (snicker) Lixis's rec room. Also when Lixis has someone else on his ship (who was welcomed) he has a special guest room for them.
Weaponry: The Room The Kira Pirates keep their weapons in, and where they create them.
Description: a very dirty room filled with weapons of all kinds, there is an anvil in the middle of the room where the blacksmith makes weapons.
Armory: The Room where The Kira Pirates keep their armor and defenses.
Description: A room which is rather clean unlike the weaponry. Everything there is shining and clean.
Public Rec Room: The Room The Kira Pirates
Description: a miniature casino inside of the massacre which is filled with arcade games and gambling machines. A very bright room.
Lixis's Rec Room: The Room where The Kira Pirates's captain Lixis relaxes.
Description: A Dark room with lots of pictures in frames of Roronoa Senshi. Also Lixis writes his love letters to her there.
Guest Room: The Room where The Kira Pirates keep their guests.
Kitchen: The Room where The Kira Pirates cook their (delicious) meals.
Sea Level: The Room where The Kira Pirates punish df-using prisoners, and the non-df users in the Kira pirates relax in the seperate area of the level.
Prison: The Room where The Kira Pirates keep their prisoners.
Animal Storage Room: The Room Where The Kira Pirates keep their pets and other animals.
Description: The Kira Pirates have recreated an environment for all of the animals there, like an aquarium for the fish, a jungle for the monkeys and other kinds of creatures have their own environments.
Cannon Room: The Room Where The Kira Pirates keep their gunpowder, cannonballs, and cannons.
Description: A rather dusty room in the kira pirate's ship where they fire the cannons at other ships.