In Cianwood's legends, the Black Tyranitar once wreaked havoc in the Johto region many years ago. It had uncontrollable power, and it took nearly all of the Fighting Pokémon in Cianwood to weaken it before it can be sealed in an ancient Poké Ball made from a Red Apricorn in the backyard of the Cianwood Gym. During the present, Black unsealed the Black Tyranitar in order to catch it and it went on rampage again.
Gold tried his best to fight the Black Tyranitar and seal it back in, but he couldn't do it on his own. Eusine was one of the people who helped out Gold. The Black Tyranitar destroyed nearly all of Cianwood City before Jasmine arrived with the citizens of Olivine City to help. The Black Tyranitar made a huge jump in an attempt to land on Olivine City to destroy the rest of the Johto region. Gold requested the help of a Lugia that he had befriended earlier. Lugia stopped Tyranitar on its way and fired a powerful attack, which sent Tyranitar back all the way to Cianwood City. It fainted when it landed back on Cianwood City.
Gold attempted to throw a Level Ball, but it bounced off, signifying that even with the combined levels of all the Pokémon presented (including the citizens of Cianwood and Olivine City), the level of the Black Tyranitar was too high. Black, on the other hand, was reluctant to send out his Pokémon. However, he eventually sent out his Pokémon as the Tyranitar made a fool out of him. With the level of Black's Pokémon added in, the Level Ball finally captured the Black Tyranitar and saved the Johto region. It was later placed back under the seal in the Cianwood Gym backyard.
Personality and characteristics
The Black Tyranitar is extremely violent and powerful, being compared to Legendary Pokémon in terms of power. It's also capable of incredible athletic feats, as demonstrated when it almost managed to leap straight from Cianwood to Olivine in one jump.
For more information on this Pokémon's species, see Tyranitar.