The Dark Ball is an movie-exclusive item, appearing in Pokémon 4Ever - Celebi: Voice of the Forest.
The Dark Ball is a unique Poké Ball. It is known to augment a Pokémon's abilities, allowing them to surpass the limits of the original Pokémon's powers by bringing a Pokémon to its highest level. The Dark Ball is also able to bring under control Pokémon that already belong to a Trainer, as well as rendering it utterly void of its free will. However, it is also noted that a particularly strong-willed Pokémon can break free from the control of a Dark Ball, but the repercussions can be equally life threatening for the Pokémon. Pokémon under the influence of Dark Balls also bear similarities to Shadow Pokémon. Dark Balls are also shown to alter the target Pokémon's colour; in which their skin tone is darkened.
A primary example when a Dark Ball was used includes when the Iron Masked Marauder interrogated the poacher who discovered a Celebi 40 years ago. Since the poacher refused to show him where the Celebi was found, the Iron Mask Marauder captured a Tyranitar belonging to the poacher with a Dark Ball. When released from its Poké Ball, the augmented Tyranitar proceeded to decimate the entire poacher's base, prompting the poacher to comply. It was also used to capture the Legendary Pokémon Celebi, using it to form an enormous grass titan. Whilst captured by the Dark Ball, Celebi's eyes become cold and turns on Ash and Sammy. When it broke free from the Dark Ball's influence, the strain was too much and Celebi was unable to be restored. Sneasel & Scizor were also captured using Dark Balls.
Trivia
During Pokémon Diamond and Pearl, the Dusk Ball was called the Dark Ball in Japanese versions, but was renamed as Dusk Ball in English localisations in order to prevent discrepancies between the two.
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