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Dark Rust (Japanese: くろのラストBlack Rust) is a living Wonder Key and the embodiment of ruin that appears in Pokémon Rumble Blast. It commands the Dark Legion, which is made up of RustyToy Pokémon. Dark Rust is not a Pokémon, and it is the final boss of Pokémon Rumble Blast.
After defeating Cobalion at Fallaway Edge, his nearby followers begin to rust, leaving him wondering if his actions were unjust. At that moment, Dark Rust appears and reveals itself, as well as its intentions on making Toyland and its residents rust away. Dark Rust then disappears into the World Axle, where the player and Cobalion give chase.
After making their way through all the challenges that were found in the World Axle, the player and Cobalion finally reach World Axle - Deepest Level, where Dark Rust awaits. After entering the fort, the player and Cobalion see Dark Rust below, where it prepares to attack the heroes with its surrounding Rusty Pokémon and plunge the world into darkness. Cobalion then tells the player that he has a plan, but the player needs to stop the Dark Legion in order for it to work. The player then enters the battle, where they fight their way through five chambers until they get to Dark Rust itself.
The final battle commences, where Dark Rust proves to be invincible due to the barrier it creates. Cobalion begins his plan by placing his Wonder Key on his head and leaping down onto Dark Rust below. The powerful attack connects, destroying Dark Rust's barrier and disabling its ability to create another one. With the barrier gone, the battle resumes, where the player emerges victorious, defeating Dark Rust. Dark Rust then falls apart, never to be seen again and cleansing the World Axle of its rust, refilling the Glowdrop fountains once again.
Spoilers end here.
It is able to control Toy Pokémon by "Rusting" them. Rusty Pokémon become members of the Dark Legion.
In the final battle, it will enclose itself in a barrier every time its HP drops to half. The barrier blocks all attacks and restores its HP until it is at full health. This ability is removed halfway through the battle due to Cobalion's sacrifice to break it.
It is able to use a surrounding wave attack, despite being neither a Pokémon nor a Toy Pokémon.
It can shoot four projectiles in front of it three times in succession, or it can shoot them all around itself once. These projectiles may lower defense.
Like most bosses, it can summon minions. It summons Sigilyph and Unown.
Dark Rust's type is Psychic.
It is similar to the Bittercold, Dark Matter and the Void Shadows in that they can be fought, but are not Pokémon.
Dark Rust is also the only one of these to be found in a series other than Pokémon Mystery Dungeon.
Despite not being a true Pokémon, Dark Rust has its own cry.
Even though it was shown to have been destroyed, Dark Rust can be rebattled whenever the player re-enters Dark Rust's Fort.
However, neither its barrier nor the cutscene with Cobalion reappear in the rematches.