Lake Valor is one of the three legendary lakes of Sinnoh, which is where Azelf is located. It is located northeast of Pastoria City.
Team Galactic's Plot
This is where Team Galactic captures Azelf, the second Pokémon needed to create the red chain. They use a Galactic Bomb, a bomb with a horrifyingly forceful blast, to clear the water out of the lake. Once you enter the cavern, you battle Commander Saturn. Even if you win, Team Galactic still makes off with the Pokémon, completely disregarding the large amount of Magikarp that are flopping around on the lake bed, their lives endangered.