Mata Nui II

The Mata Nui II, formerly known as Impel Down, is a large underwater complex that is home to several colonies of Matoran. It is located on the planet of Impel Down, and was previously a prison of the same name owned by the Galactic Government until it was overtaken by forces belonging to Blackout. As a result of the First Battle of Impel Down, the remainder of its original occupants plus the forces of Blackout and Zev deserted the prison, prompting the Matoran to claim it as their new home and redesign much of it to serve their purposes.

History of Impel Down
Impel Down was originally run by the Gaolers, powerful prison guards; the Chief Warden Magellan, a giant gargoyle with the power to control poison; Vice Warden Hannyabal, a devil-man with a face shaped like a Japanese hannya mask; and Chief Guard Sadi-chan, a she-devil wielding a pitchfork with a concealed whip. Impel Down had an entrance level, where visitors were checked before being brought in, and five main levels.

The Last Secret of Impel Down
Long ago in Impel Down, there was no Vice Warden. Instead, there were two Chief Wardens: Magellan, and the much more dangerous Shiryuu. Both were equals in power, but Magellan's working hours were limited by the gastrointestinal aftermaths of his eating of poisonous foods, so Shiryuu was considered much more dangerous. Shiryuu's appearance was of a gigantic man, with sandy blonde hair, a broad nose and chin, a black suit with a long white coat draped over his shoulders, a thick buck cigar constantly lit in his mouth, and a proportionally sized katana at his side. One day, while Magellan had been blessed by a rare reprieve from his stomachaches and likewise, he was on patrol in Level Two, where he discovered a grisly sight: Several dismembered, disemboweled or just beheaded prisoners strewn about, laying in pools of blood and guts. As he got deeper, he discovered Shiryuu, blade soaked in the blood of the prisoners. Shiryuu had always been sadistic and believed the prisoners to be no more than trash, but this was simply crossing the line. Shiryuu was narrowly defeated in battle by Magellan, and sentenced to death in Impel Down, but given a postponed sentence so that he could rot indefinitely until the G.G. (Galactic Government) decided that the execution of a vicious criminal would boost their popularity. He is currently in Level Six, the Eternal Prison, where he may stay forever. However, the flames of anger and hatred haven't even dimmed in Shiryuu's heart; in fact, they have gotten ever stronger, and if given half a chance, he could very well break out of his cell and go on a rampage even worse than the one that got him imprisoned.

Downfall
During the final phase of Blackout's plan to take over the galaxy, he invaded Impel Down, the first invasion of the planet since the Shadow Proclamation's war with the Vrai many thousands of years ago, and the only successful one ever. It was extremely carefully planned, as Blackout had spent some time as a Shadow Proclamation operative, and knew everything about the prison. Two neutron bombs were teleported to the surface, destroying Level Six and killing the prison's wardens. The moment the planet's automated defence turrets locked onto Darkmount, the Makuta leader's city-ship, the defence mainframe was deactivated by a computer virus planted in the system. Before the command center could be evacuated, it was completely annihilated by a salvo of fusion bombs. With the entire planet completely defenceless after the initial attack, Blackout and a squad of Rahkshi teleported to the surface and began freeing the prisoners, intending to transport them to various planets and cause chaos all over the galaxy.

However, as revealed once by Zev in passing, the Top Gaolers and Shiryuu of the Rain somehow escaped. Apparently, somebody gave them a warning and instructions on how to escape; if one were to thoroughly examine Level Six, they would find that no significant residues of living creatures are left for all of those prisoners to have been killed -- Essentially, that kind of blast in such close proximity would have fossilized them. Also, Zev's words are revealed to be true, as Shiryuu has made a few bloody entrances onto the stage.