Provided you've challenged Ho-Oh to a battle in Pokémon SoulSilver and caused it to faint and then you ended up saving the game after the fact then you can head over to the Indigo Plateau in the Kanto region and go into the Pokémon League building in order to challenge the 4 Elite 4 Pokémon Trainers as well as the Pokémon League Champion, Lance. Once you defeat Lance, his defeat will activate the regeneration system that will allow all one-off Pokémon that have been KOed such as Entei, Ho-Oh, Suicune, Lugia, Articuno, Mewtwo and Raikou among others to regenerate in the wild with the exception of Suicune because it will regenerate in the Burned Tower.
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Nothing, you just can't catch him.
it wil never come back...
Once you kill Tornadus, you have two chances. Since it is a legendary Pokemon, there is only one of its kind, and will appear after the first one.
Well technically, you can't KILL a Pokemon. You can just make it faint. When you make a legendary Pokemon faint, it will be gone until you start a new game.
I'm pretty sure they go away permanently if you kill ANY type of legendary pokemon. well when i killed tornadus a few days later it came back