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Getting a legendary prior to gen 6
>Traverse a dungeon with several trainers and high leveled wild pokemon. There's several floors and puzzles that need to be solved, might need some HMs. At the end of the dungeon you finally get to battle the legendary which has a 3% catch rate, need to catch it normally like any other pokemon
>For event legendaries, you still need to travel to a specific location to battle and catch the legendary. There might be some requirements to get the legendary (eg: to get darkrai you need to be a champion and must've obtained the lunar wing from cresselia)
Getting a legendary post-gen 6
>Legendaries have high catch rates. They are found in extremely short dungeons, you don't need to battle any trainer to get to them and there are no puzzles anymore. Some legendaries are gifted to the player, no need to catch them
>Event legendaries are handed to you by some random guy at the pokemon center