POKEMON BLACK LEGENDARY POKEMON GUIDE,
Location: Liberty Island
Method: Victini is the first legendary you may encounter within the game. Victini can only be obtained via use of the Liberty Pass item that is given over WiFi. With this item, you need to go to Castelia City and enter one of the boats. This boat will take you to Liberty Island where you'll have to fight through numerous Team Plasma members in order to get to Victini. At the end, Victini will be there to battle.
#643 and 644- Rashiram and Zekrom.
Location: N's Castle/Dragonspiral Tower
Method: The next main Pokémon you will encounter are Reshiram and Zekrom. These are obtained after you have beaten the Elite Four and N's plan of attack comes into fruition. Reshiram/Zekrom will appear to you in Pokémon Black & White respectively. If, when you reach the end of the game, you have a maxed party and maxed boxes, the game will progress without you needing to capture Reshiram/Zekrom (previously, the game will not continue until it is captured, and you will be able to capture Reshiram/Zekrom in Black & White respectively in the Dragonspiral Tower. They will once again be at Level 50. Reshiram & Zekrom cannot be captured in their shiny appearances.
Location: Mistralton Cave
Method: After you have obtained surf, you'll get access to a variety of new areas. The first of note is in Mistralton Cave in Route 6. Deep within this cave, you will find a man talking about the three fighting legendary Pokémon and he'll tell you about them and then inform you that there is one at the end of the cave. Go there and you'll find Cobalion
Location: Victory Road
Method: Following on from that, near the top of Victory Road, you will find a new cave has opened up. Within this cave is the next one of these Legendary Pokémon; Terrakion. You need strength to reach it
Location: Pinwheel Forest
Method: After obtaining Cobalion, you'll now have access to a new area in Pinwheel Forest. This area is on the eastern side of the forest and will allow you to find and capture the Grass/Fighting Legend, Virizion
Location: Abundant Shrine
Method: Once you have captured and traded with a friend to get both Tornadus and Thundurus, take them both to the Abundant Shrine off Route 14. At this shrine, when you go and interact with it with the two Pokémon in your team, the Ground/Flying Landorus will swoop down and battle you.
Location: Giant Chasm
Method: North of Route 13, you'll find a new area called the Giant Chasm. When you enter, this area is filled with forestry, but when you come near the lake in the middle, you'll hear a screech and the whole area flash freezes. Go to the north and you'll find the Dragon/Ice Pokémon Kyurem in a cave.
Each of these Pokémon are relevant to their game, Tornadus in black and Thundurus in white.
Method: after you have all eight badges and head to Route 10, you'll be informed of a weird weather pattern in Route 7. Head there and continue on until you reach the second house. After being invited into a house, when you leave, you'll get confronted by Tornadus and he'll fly off. From this point, he will start roaming the areas of Unova. In the mornings, he will appear in the southern routes and in the evenings he'll appear in the northern routes. He appears in Pokémon Black.