Mata Nui II

Impel Down is an immense underwater prison, on a planet of the same name that is completely covered by ocean. It is 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 has an entrance level, where visitors are checked before being brought in, and five main levels:

Levels
Lvl. One Bloody Prison: An area filled with needles for grass and trees with blade-leaves, prisoners are forced to run by Gaolers. Thus, it is dyed crimson by all the blood.

Lvl. Two Beast Prison: Filled with deadly creatures like the Basilisks (chicken-dragons), Manticores (human-faced lions), and the worst of all, the giant, feathered, human-faced lion; the Sphinx. They chase any prisoners that get out of their cells, so prisoners don't try to escape.

Lvl. Three Starvation Prison: A desert wasteland, baking hot from the blazing level below. Prisoners are only given just enough food and water to keep alive, keeping them in a constant, painful state of half-death.

Lvl. Four Blazing Prison: A gigantic maze of hanging platforms and staircases, kept blazing hot from a titanic kettle of boiling blood below. Prisoners here are forced to do constant manual labor, and if they take breaks when not instructed, they are simply thrown into the boiling blood below. This is where Liquid Ocelot is kept; he is the only prisoner dangerous enough to warrant a separate cell

Lvl. Five Freezing Prison: An arctic wasteland, prisoners here are constantly in danger of completely freezing; they cannot even eat food properly, as all is totally frozen. Wolves, the most dangerous beasts of Impel Down, reside here, stalking prisoners; the Wolves are kept here because they attacked other beasts when on Level Two. Communication here is completely cut off from the rest of Impel Down, so even Gaolers are at risk. However, just under their noses is a secret paradise.....

Lvl. Five Point Five Secret Hall of Love: A giant network of rooms and tunnels under Level Five, this is where Zev has taken up residence after escaping Level Five. Every room is like a cross between a sit down cafe and a discotheque, and prisoners live in joyous peace here. There are connections all over Impel Down from here, a security Den-Den Mushi to watch the rest of the prison, and residents steal newspapers from trashcans to keep up on current events.

Lvl. Six Eternal Prison: This is where prisoners too dangerous or inconvenient for anywhere in society are kept. Prisoners here either serve a life sentence or are on death row. It is a dank, dark place, where hope can easily die. No one has ever escaped from here, and it is unlikely any ever will...

Notable prisoners include Zev Raregroove (Level 5.5), Liquid Ocelot (Level 4) and Maelstrom (Level 2).

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.

...

I'll give you three guesses who masterminded the escape.

Post-Great War
After the defeat of Blackout during the Battle of Impel Down, Impel Down was inhabited by the Matoran, and renamed Mata Nui II, in honour of the Great Spirit.