This monster was one of the three Legendary Dragons that protected Atlantis, and the Dominion of the Beasts.
In episode 147, Yugi Muto and Yami Yugi were chosen to release Timaeus from his 10,000 year long slumber. This card appeared in Yugi's deck when he was attacked by the Great Leviathan. Yugi then took this card out and summoned it to protect him from the Great Leviathan. Timaeus then defeated the Great Leviathan, but did not destroy it as it retreated away after Timaeus defeated it.
In episode 148, the gang listen to the news regarding the Great Leviathan attacking Domino City, Joey Wheeler notes that this card was able to defeat the monster before it could do any actual harm, but only for now. Tristan Taylor then takes the card and notes he doesn't know how this card was able to do all that since it is just a card. Joey takes it back and states that's only if you give it a first impression. He notes any real Duelist would know this card is special so it has to be handled with care. Joey then hands the card back to Yugi and asks if that is right. Yugi agrees and states there is no telling what else this card can do. Yami Yugi agrees as this card contains the spirit of an ancient dragon so they should be careful. The gang then try to figure out what to do now about these mysteries, but Yami Yugi notes the secret should be in this card. Joey then takes the card to have a look at it. He notes if they can this monster to come out, then maybe he can tell some answers about the recent events that have occurred. Tristan Taylor then hits Joey and states he thought Joey said to handle it with care. Joey and Tristan then got into a fight until Yugi's grandfather interrupted with a package.
This card also appears in a flashback that Dartz has when he remembers Yami Yugi using this card to defeat the Great Leviathan's eye and temporarily leave it incapacitated.
In episode 149, Yugi makes note to his friends about the recent events that have happened to them and shows this card to his friends. He then states that this card is the only protection they have against the Great Leviathan, but even that doesn't seem like enough because the Leviathan is still out there somewhere.
In episode 150, Seto Kaiba releases "Critias" from his icy imprisonment. Shortly after that happens, Yugi, who was holding this card, notices this card glowing which was a signal to Yugi that another Dragon was released.
In episode 152, during Joey's Duel against Mai Valentine, Yami looks at this card and notes in his mind this card has the power of defeating the "Orichalcos", but he and Yugi are not sure how it will save Joey.
In episode 153, Joey ran out of options to survive his rematch with Mai, but his Deck began to glow. Yami Yugi and Yugi then noticed their Deck glowing. They then take the glowing card out and see that it is this card, signifying that the third chosen Duelist has been chosen.
In episode 154, all three chosen Duelists (Yami Yugi, Kaiba, and Joey) came together in Industrial Illusions and each showed their respective Legendary Dragons. All three cards then begin to glow since the chosen Duelists have now united. They then hold their respective cards up into the air. In the next episode, the glow disappears and the three Duelists lower down their cards before Kaiba and Mokuba leave.
In episode 157, Yami Yugi uses this card during his Duel against Rafael. He activates this card to merge it with "Dark Magician Girl" in order to summon "Dark Magician Girl the Dragon Knight". Unfortunately, Yami Yugi is given "The Seal of Orichalcos" due to Rafael activating "Exchange". Yami becomes tempted to use it, but he tries to use his willpower to prevent himself from playing it, knowing how evil the power of "The Seal" is. However the temptation of using "The Seal of Orichalcos" eventually becomes too much for Yami Yugi to delay as he continues to take damage while Rafael takes almost no damage. When he becomes close to losing, he plays "The Seal of Orichalcos" against Yugi's better judgment. In the next episode, Yami Yugi's action destroys "Dark Magician Girl the Dragon Knight" and replaces it with "Dark Magician Girl" due to an saddened Timaeus forced to abandoning Yami Yugi for playing "The Seal of Orichalcos". Playing "The Seal of Orichalcos" also caused Yami to lose control of his rage, anger, and emotions, which eventually lead to Yami Yugi losing the Duel. After the duel, Yami Yugi can't use Timaeus right now until he could control his anger.
In episode 162, Yami Yugi uses this card during his Duel against Weevil Underwood. Yami Yugi attempts to fuse this card with "Poison Butterfly", but Timaeus apologizes to Yami Yugi and destroys himself due to him "abandoning" Yami Yugi for playing "The Seal of Orichalcos" previously, causing the Fusion to not work which Atem wasn't ready to destroy the darkness within him. Yami Yugi was then forced to defeat Weevil without Timaeus' help by using Berserker Soul.
In episode 163, Ironheart picks this card up from the ground after Yami Yugi drops his Deck while being flung from the train after defeating Weevil. Ironheart notes to Yami Yugi since he possesses this card, he must be one of the chosen Duelists. Yami Yugi reveals that he can't use it anymore due to his actions from a previous incident. Ironheart then tells Yami Yugi that he will hold onto it until the time was right to give it back to him. After Yami Yugi was forced to challenge Yugi in the Stone Wastelands and defeat him, Timaeus actually help Atem to conquer the darkness at once and for all.
In episode 164, Yami Yugi was forced to Duel one of Dartz's Orichalcos soldiers. He nearly loses, but Ironheart, Skye, and Chris sacrifice themselves to give Timaeus back to the Pharaoh before Dartz strikes them down. Yami Yugi then activates this card to merge it with "Dark Magician Girl" in order to Special Summon "Dark Magician Girl the Dragon Knight". Rafael, who was observing the Duel, was greatly surprised that Yami Yugi was able to play and use this card again due to what happened in their previous Duel.
In episode 167, Kaiba defeats Alister, but he is unable to stop the plane from crashing due to what "The Seal of Orichalcos" did. All three Legendary Dragons then came out of their owner's Decks as actual monsters. They then gathered around the plane and brought it down to the ground safely.
In episode 168, Yami Yugi and Kaiba confront Dartz at the KC USA building. They then summon their respective Legendary Dragons against Dartz and the Great Leviathan. Dartz asks Timaeus how his eye is before Timaeus lets out a roar. Timaeus attacks Dartz and the Great Leviathan, but they escape. Shortly after, both Legendary Dragons were recalled back by their masters.
This card also appears in a flashback Dartz has when he remembers this card defeating the Great Leviathan's eye and scattering the shattered pieces of the Leviathan across the globe.
In episode 170, Yami Yugi and his friends get surrounded by Orichalcos Soldiers, but Yami Yugi and Kaiba summon their Legendary Dragons and use them to defeat the many soldiers. Rebecca Hawkins and Téa Gardner get attacked by some soldiers, but Yami Yugi orders Timaeus to destroy them. After the soldiers are cleared out, Yami Yugi and Kaiba recall back their Dragons. Even when the gang try to assure themselves that they will find Joey through his Duel Disk's location, Yami Yugi looks at this card as he is concerned about what will happen to Joey when he faces one of the three remaining people that are still helping Dartz.
In episode 171, Joey plays "Claw of Hermos" during his Duel against Valon. This causes this card to glow. Yami Yugi and his friends notice this and see that this signifies that Joey played his Dragon. After "Rocket Hermos Cannon" is destroyed, the glow around this card fades which makes Yami Yugi and his friends concerned over what happened to Joey.
In episode 172, Téa Gardner states to Tristan and Yami Yugi that Joey is nearby and that they should look down each street, Yami Yugi looks at this card, pondering over what happened earlier. Yami Yugi explains to Téa that he could sense the presence of Joey's Legendary Dragon earlier, but it's now gone and so he guesses that means Joey's Dragon was destroyed. After Yami Yugi and his friends see a vortex of light coming out from beyond some buildings, Yami Yugi notices that this card is glowing again, signifying Joey has played his Legendary Dragon again. Later Yami Yugi takes this card out and looks at it. He soon decides to use this card in order to lead himself, Tristan, and Téa to Joey. He then takes off on a path and requests his friends to follow him.
This card also appears in a flashback Yami Yugi has when he explains to Tristan and Téa that he earlier sensed the presence of Joey's Dragon through his own, but it faded out soon after, heavily suggesting that Hermos was destroyed.
In episode 173, Joey loses to Mai. Yami Yugi then stops for a brief moment and looks at this card. He then looks up and sees this card on top of a building, roaring in sorrow before dissolving away. This signifies that Timaeus has learned that Joey and Hermos have lost to the Orichalcos.
In episode 175, Yami Yugi uses this card during his Duel against Rafael. He activates this card to merge this card with "Dark Magician" in order to Special Summon "Amulet Dragon". In the next episode, Yami activates "Altar of Restoration" to return this card from his Graveyard to his hand. He then activates this card to merge it with "Dark Magician Girl" in order to Special Summon "Dark Magician Girl the Dragon Knight".
In episode 182, Yami Yugi uses this card in the battle with The Great Leviathan. After all three Chosen Duelists summoned their Legendary Dragons, Yami Yugi used "Legend of Heart" to transform the Dragons into the Legendary Knights.
In episode 184, Yami Yugi was able to defeat Dartz and The Great Leviathan. Once that was done, Timaeus returned home with his friends. This card along with the other Legendary Dragon cards dissolved away when this happens as the Legendary Knights no longer needed to assist the chosen Duelists or their comrades with the Orichalcos and the Great Leviathan defeated for good (This card was not shown dissolving away, but the fact that "Claw of Hermos" and "Fang of Critias" dissolved away after the Legendary Knights left back into the Dominion of the Beasts proves that this card dissolved away too).