To get Slifer, go to the Domino Station. Talk to the people that are guarding the entrance. Tag team with Yugi or Joey, then go on the train. Talk to everyone twice and defeat the person that is standing in front of the next car. Defeat the person that comes up to you. Exit the train. Go into the Egyptian exhibit. Talk to the person standing in front of the stairwell. Go down the stairs and challenge the person to a duel. If you defeat him, he will give you the Millennium Necklace. Go back to Domino City and talk to Ishizu. Go to the Domino pier and go into the boat. Talk to Mai and defeat Jean Claude Magnum in a duel. Mai will open the door to the cargo hold. Challenge the person there to a duel. If you defeat him, he will give you the Millennium Key. Go back to Domino City and talk to Ishizu and she will use the two Millennium items to resurrect Slifer the Sky Dragon.