After you arrive in Nibelheim, go to the Shinra Mansion after you explore the town. Be careful in the mansion as the enemies here tend to be annoying and some are quite powerful. Once you enter through the main door, go upstairs and head to the left of the balcony and into the room with the safe. In there is a safe. The combination is (Right 36), (Left 10), (Right 59), (Right 97). Now, I'm warning you, the boss you are about to fight is possibly one of the most difficult bosses in the game. Make sure you have your best equipment and your best materia equipped before you attempt to fight this boss. It is also a good idea to save here. Now turn the dial to the correct numbers as fast as possible and avoid going over that number as that counts as a failure. When you get to the number, press the action button. If you do it correctly, the "Odin" materia will pop out and you will be attacked by the boss, "Lost Number". After you beat the boss, you recieve the "Basement Key". After you pick up the "Odin" materia, go to the right of the balcony and into the hidden staircase. Descend and travel along the tunnel until you see a door. The first door you see is the one you want. Go inside and talk to the coffin. Vincent will pop out and talk to you. He then goes back inside the coffin. Talk to him until he refuses to come out of his coffin. Then head out and into the other door in the tunnel where you will meet Sephiroth. After the encounter, head out of the tunnel. Vincent stops you and joins your party.