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.
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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
Nothing, you just can't catch him.
it wil never come back...
if by deciples you mean legendarys then reshiram ans zekrom are in n's castle bugt if you kill them they are in dragonspiral tower, victini is an event, tornadus(black) and thundurus (white) is found at route 7 after beating all 8 gyms. Cobalion is in mistralton cave, virizion is in pinwheel forest and terrakion is in victory road. Landorus is in abundant shrine after beating N and Ghetsis and getting both thundurus and tornadus in one game (trade with someone or cheat). Kyurem is in giant chasm after beating n and Ghetsis.
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.