From Bulbapedia, the community-driven Pokémon encyclopedia.
A Dark Ball is a special type of Poké Ball which appeared in Pokémon 4Ever.
The only person who has been seen using a Dark Ball is Vicious. He claimed that any Pokémon captured with a Dark Ball will become evil and their power will increase to the highest level. The Pokémon loses its soul and obeys all commands of its master.
The control of a Dark Ball can be fought and even broken if the Pokémon has enough will power. Should this happen, the Dark Ball that captured them will be destroyed. This happened to Celebi after it had been captured by Vicious. However, the Pokémon nearly lost its life in the process. Thankfully, other Celebi from the past and future were able to save the Pokémon. Presumably, Tyranitar, Scizor, and Sneasel were able to break free, too. They were all seen going into the forest to be normal Pokémon at the end of the movie.
Another Pokémon that was captured by a Dark Ball was a Tyranitar that belonged to a Pokémon Hunter. Vicious used his Dark Ball to capture it and bring it under his control. He also used Dark Balls to capture his Scizor and Sneasel.