Slifer the Sky Dragon, known as Sky Dragon of Osiris in the Japanese version is a character version of the card "Slifer the Sky Dragon". Slifer appears in the manga and anime, as one of the most powerful monster spirits, the Egyptian Gods. In the Japanese anime and manga, its card is called "Saint Dragon - The God of Osiris".
When Atem spotted Thief Bakura fleeing from the palace after he had attacked Aknadin, he chased after him on horseback and Summoned Critias to fight Bakura's Diabound. Atem allowed Critias to be struck in order to protect innocent civilians from Diabound's attacks and then got Critias to force the battle into the sky. However Diabound was able to disappear into the darkness and attack Critias from behind. Atem pretended to surrender his Millennium Pendant in order to distract Thief Bakura while he had Critias sneak up on him. However Bakura got Diabound to make a similar sneak attack on the Pharaoh. After the priests arrived, Atem sacrificed Critias, so that Seto's Duos could attack Diabound, but Duos wound up hitting a decoy ka instead. By defeating Critias, Diabound gained its power, including its attack.
3000 (5000 in the Dub) years later, Maximillion Pegasus created cards depicting the three legendary dragons, for his game Duel Monsters. The souls of the legendary dragons came to dwell in the cards. On realizing this, Pegasus regarded their creation as one of his greatest mistakes. He tried to destroy them, but could not bring himself to do it, so he had the Egyptian supreme council of antiquities bury the cards in different locations in the Valley of the Kings. The Ghouls managed to steal the "Critias" and "Timaeus" cards. However the Egyptian supreme council of antiquities was able to prevent them finding the "Hermos" card. Ishizu Ishtar, from the council, later gave it to Seto Kaiba, so that he would host the Battle City tournament to retrieve the other two.
The leader of the Ghouls, Marik Ishtar entered the Battle City tournament with the "Critias" and "Timaeus" cards. Using Strings as his puppet, he Dueled Yami Yugi in the preliminaries and used "Critias" in the Duel. However Yami managed to win the Duel and won "Critias" through the tournament rules.
Yami Yugi used "Critias" in his semi final Duel against Yami Bakura. Summoning it and attacking Yami Bakura became Yami Yugi's only way of winning the Duel. Marik exploited this, by having Yami Bakura remove his control over Ryo Bakura's body. Ryo, who was injured and significantly weaker than Yami Bakura would not be able to withstand an attack from Critias, making Yami Yugi reluctant to attack. However, Yami Bakura took back control of Ryo's body and urged him to attack, assuring him Ryo would be okay.
During Yami Yugi and Kaiba's semi final Duel, they got "Critias" and "Hermos" to battle, which caused Yami and Kaiba to see a vision of an ancient city. Here Critias and Hermos were turned to stone, while Priest Seto and Pharaoh Atem battled. Yami used Critias in his final Duel against Yami Marik.
In order to regain his memories, Yami Yugi presented the three legendary dragons to the stone tablet in the museum. However, the legendary dragons began to crackle with black energy as Dartz drained their power. Yami was then interrupted as Duel Monster spirits began appearing throughout the city. Later that day, Dartz's men broke into Kame Game and stole the legendary dragons. Dartz placed the cards in the mouths of three stone serpents in order to use their power to summon The Great Leviathan.
After defeating Dartz in a Duel, Yugi retrieved the legendary dragons, though their strength had already been drained. After Yugi's soul had been absorbed into The Great Leviathan, the souls of every male Dartz had captured were able to lend him their strength. With it, Yami called out, "In the name of the Pharaoh, awaken!" and summoned the three legendary dragons. By summoning them, Yami was also able to free himself, Joey Wheeler and Seto Kaiba from the Leviathan. Hermos, Critias and Timaeus engaged in battle with The Great Leviathan. The three combined their attacks and seemed to defeat it at first. However the Leviathan reappeared in a different form, which Yami managed to defeat.
Pyramid of Light
Kaiba Dueled and defeated Pegasus to obtain a card capable of defeating the legendary dragons. Anubis added an extra card, "Pyramid of Light" to Kaiba's prize. Kaiba went on to face Yami Yugi in a Duel and used "Obligatory Summon" to force him to Summon the legendary dragons and then activated "Pyramid of Light", destroying the legendary dragons.
When Yami Yugi entered the Memory World, he relived some of his life as Pharaoh Atem, and fought the battle against Thief Bakura, in which he summoned Critias, again. The Pharaoh used the three legendary dragons to battle Zorc Necrophades. However Zorc managed to overpower them and turn them into stone. Upon relearning his name, Atem was able to summon the three legendary dragons again, this time from the cards in Yugi's Deck. He then sacrificed them to call forth The Creator of Light, Horakhty, who defeated Zorc.
Yugi and Atem constructed separate Decks for the Ceremonial Battle, which Atem needed to lose in order to pass on to the afterlife. Despite needing to lose, Atem gave the Duel his all. He included the three legendary dragons in his Deck and managed to summon the three of them at once in the Duel. However Yugi was able to destroy them and win.