The Legend says:

If you go to Mt. Silver on the day 16 of October and go to the edge of the first lake in Mt. Silver, the banished Silver and Red bird-like Creature will arise from the deep waters of Mt. Silver to get revenge from the Rainbow-colored Phoenix that banished it.

A brave girl called Kanon went to Mt. Silver on that day to face the creature, but for her misfortune, the creature was waiting for her. She brought her best Pokémon with her, especially her long and best friend, a yellow and white Dragon Pokémon called Latias.

She faced the creature and saw that she had a stronger opponent than she expected. She was unable to weaken it without the help of Latias. In the end she was forced to use the Ultimate Master Ball to seal the creature. The creature was sealed in the Master Ball, never to be released again. [link] This is the link of the video of the battle.