Castle Bleck is the setting of Chapter 8 in Super Paper Mario, and it is found after entering the Black Door on Flopside Tower. Located in a realm of darkness, it is Count Bleck's home. The castle is separated into four main areas, one for each Chapter section respectively: the Entry, the Foyer, the Interior, and the Inner Sanctum. Castle Bleck is one of the only two worlds (the other being The Bitlands) to have at least one boss battle in each level (O'Chunks in 8-1, Mimi in 8-2, Dimentio in 8-3, and Count Bleck and Super Dimentio in 8-4). Castle Bleck's theme is a cover version of the Pure Hearts', Count Bleck's, and the Chaos Heart's theme, mixed with some 8-bit sound effects. According to The InterNed, the music that plays while Peach or Luigi are trying to escape Castle Bleck is titled "In the Darkness".
After Mario collects the Pure Heart in the Yold Ruins, Princess Peach finds herself lost within the castle. With the help of Private Koopa, she attempts to find a way to escape. However, when she finds a dead end, Nastasia catches up to her, and is about to place her under hypnosis when Dimentio intervenes, unseen, and teleports Peach out of the castle.
Later, after the Pure Heart in Merlee's Basement is found, Luigi also finds himself within Castle Bleck. With the help of a pair of Goombas, he attempts an escape just like Peach. Once again, a dead end is found (in actuality, this is the beginning of Chapter 8-2, Castle Bleck Foyer, where an exit isn't found), and Luigi is taken captive and turned into Mr. L by Nastasia's hypnotic powers.
When Mario and his party finally collect all eight Pure Hearts, they are finally able to access Castle Bleck via opening the Black Door from Flopside. In Chapter 8-1, Bowser and O'Chunks face off against each other. When Bowser wins, the ceiling begins to fall; however, O'Chunks holds it up because of his honor code. Bowser stays behind, even as the two keep telling one another to leave. The ceiling presumably crushes them both.
In 8-2, Mimi repeatedly tricks the heroes before they finally confront her. She taunts Peach until the princess demands a one-on-one fight with her. After Peach wins, the floor begins to collapse due to tremors. Peach manages to hold onto the floor, and even prevents Mimi from falling to her doom. She loses her grip, however, and they fall down.
In 8-3, the player encounters several rooms with mirrors as well as countless Dimentio clones. When they finally reach the end, Mario and Luigi end up chasing after Dimentio through one area of the previous seven worlds. Once they catch up with him, Dimentio reveals his agenda against the Count and suggests that they team up so that all of them could control the Chaos Heart. Mario and Luigi, however, refuse. Dimentio then plans to attack both of them, but, when he insults Luigi's mustache, Luigi decides to defeat him on his own. When Dimentio is defeated, however, he presumably ends the game of both himself and Luigi.
Mario, now on his own, goes through the maze of the final area of Castle Bleck. He eventually reaches Count Bleck's chambers, specifically the meeting room visible in the between-chapter interludes, where he finally confronts Count Bleck and Nastasia. After Tippi and Bleck argue, Bleck sends Nastasia away and begins to fight Mario. At first, Mario's attacks have no effect; however, it is then revealed that Bowser, Peach, and Luigi all survived, as they enter the chamber. The Pure Hearts disable Count Bleck's barrier, and, together, the four heroes defeat him.
Nastasia reappears, and Count Bleck tells the heroes to finish him off so that The Void will disappear. However, Tippi is reluctant to do so. They had only just been reunited, and she didn't want them to be ripped apart so soon. At that moment, a supposedly fatal blast of energy is sent toward Count Bleck, but Nastasia takes the blow instead. Dimentio appears, saying that the Chaos Heart would not disappear if he continued to control it. It is revealed that, after Luigi had defeated him, Dimentio had planted a mind-control seed in his subconscious, and it sprouts into a Floro Sprout. Dimentio then sends Tippi and Count Bleck to Dimension D, and continues with his plans. The Chaos Heart, Dimentio, and Luigi merge, creating a freakish creature reminiscent of both Brobot and Dimentio, called Super Dimentio. Mario, Bowser, and Peach are currently unable to damage it.
Meanwhile in Dimension D, Tippi argues with Bleck that they have to do something to help Mario. Bleck, however, says that it's a pointless endeavor. They exchange a few more words before Mimi and O'Chunks appear, lending the count their support. The Pure Hearts, thought to have been wasted on Bleck, reappear, and Tippi takes them to Mario. Super Dimentio is able to be damaged now, and the final showdown commences.
Mario, Peach, and Bowser prevail, and Super Dimentio is defeated. However, in addition to Luigi's normal self, Dimentio leaves behind one fragment of his power, so that The Void would still destroy all worlds even in his absence. Count Bleck surmises that another set of Pure Hearts must be created to destroy the Chaos Heart. Everyone goes through a final door, and they end up at the altar where Peach and Bowser were forced into marriage at the start of the game. Bleck and Tippi go up to it, and proclaim their love for each other. In their supposed final moments, they are married, and the Chaos Heart is stopped. In light of the sacrifice of Timpani and Bleck, the latter's minions split up to try and make the multiverse a better place, abandoning Castle Bleck.
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