Fifth Floor

The Fifth Floor refers to a floor of Peach's Castle only accessible after defeating the Wario Apparition in Dire, Dire Docks Village. To access it, you have to BLJ into an inaccessible painting outside the Fourth Floor's map, while also having the W Key and DK unlocked in Bob-omb Battlefield. The floor itself consists of multiple paintings leading you to every village in the game excluding Eternal Village, which you can only get after the Waluigi Asparagus boss fight. The paintings on the second part of the Fifth Floor lead you to the 4 new levels, Dark Heaven 2, Robo Station Way, Burning Meteor, Stanley's Showdown, and Reginald's Last Stand. Beating the Fifth Floor unlocks Stanley and the Robo brothers as playable characters, and unlocks the Third Basement.

Waluigi Asparagus
The Waluigi Asparagus is similar to the Wario Apparition in size and appearance. If you see it, DONT TURN YOUR CONSOLE OFF. The Waluigi Asparagus will actually give you a free star and some tips on avoiding the harmful anomalies. The boss fight can be activated if you try to harass and attack the Waluigi Asparagus however, and is much harder than the Wario Apparition one as it takes place in the Internal Plexus Village. Beating him unlocks Eternal Village but at the cost of infinite guilt for killing Waluigi Asparagus. Killing him also secretly activates Ultra Hard Mode Plus and puts you on the Genocide route of the game.

Robo-Luigi
Similar to Robo-Mario, Robo-Luigi is yet another bossfight that copies Luigi's movements instead of Mario's. You have to punch his back to defeat him. Robo-Luigi has a Flaming Hot Smoke attack, and he is floaty. After being defeated he will explode and drop a secret star. Defeating him doesn't do much, but if you haven't beaten Robo Station Way, his death can make the Robo-Mario fight much harder. He randomly appears in Fifth Floor B once it's unlocked after getting the first star of Eternal Village. His Death can also activate Ultra Hard Mode Plus X, which is a harder version of the Ultra Hard Mode Plus activated after killing the Waluigi Asparagus, even though it does not announce both modes in-game.

Reginald's Last Stand
If you beat the Stanley boss and unlock him as a character you can return to the Fifth Floor and defeat the true final boss of Super Mario 64 known as Reginald. Reginald is Stanley's evil twin brother who is grey and evil and works with Dark Matter, the boss of the Third Basement. His attacks include shooting dark lasers at you, summoning evil Marios and Luigis, and draining your power meter by healing him. In the second phase, he uses Dark Matter's powers to gain giant dark hands like Bouldergeist and crush Stanley, as well as punch him. Defeating him unlocks the Third Basement B, an extension of the Third Basement.

Robo Station Way
Robo Station Way is a factory-based level located in the Fifth Floor that houses the Robo-Mario fight. The stage itself consists of mechanical enemies like Heave-Hos and Chuckyas, and also as assets from Tick-Tock Clock and Wet-Dry World, recolored to be greyscale. The level also has purple ! Blocks that summon the Green Demon when broken, which is a purple 1-Up that endlessly chases Mario and kills him when touched. Fwoosh's appear on the higher parts, recolored to be more like smoke clouds instead of normal white clouds. The level is theorized to be Wet-Dry World after Robo-Mario took it over and enslaved the inhabitants to make his factory, hence the beta name of the level being Wet-Dry Zone Bad Future, as time travel was initially planned for Super Mario 64DD. The Robo-Mario boss fight is similar to the Stanley one, where he can use all of Mario's moves, but he can also extend his arm to punch Mario and can shoot homing Amps. In the harder version after killing his brother, he can also spin around at Mario and summon Bullet Bills.