Clockwise, from top: "The Fang of Critias", "The Eye of Timaeus" and "The Claw of Hermos", in the artwork of "Legend of Heart"
"Legendary Dragon" is an archetype comprised of the dragon forms of the three "Legendary Knights" who used to live in the Dominion of the Beasts and answered Ironheart's call to defend Atlantis. Dartz transformed them into dragons to weaken them. The great ancient war ended in a draw, with the Dragons sealed away in their realm and the Leviathan severely weakened and diminished to a shadow. Their names come from the ancient philosopher Plato's workings. Despite lacking "Legendary Dragon" in its name, "The Eye of Timaeus" includes a name condition that causes it to always be treated as "Legendary Dragon Timaeus".
In the Waking the Dragons arc of Yu-Gi-Oh!, Yugi Muto, Joey Wheeler and Seto Kaiba each receive 1 of the 3 Legendary Dragon cards: "The Eye of Timaeus", "The Claw of Hermos" and "The Fang of Critias", respectively. According to Kaiba, the Dragon he received, "Critias", seemed just as powerful as the Egyptian God Cards. Another interesting note about these cards if their name corresponds to where they were stabbed and sealed during the war. Although their names are known prior to their release in the English anime, in the Japanese anime, no one, not even the monsters, knows the names of these dragons except for their chosen Duelists who learn their name upon releasing them, without even being told what their names are. Thus, they are referred to as the Nameless Dragons in the Japanese anime sometimes even by their chosen Duelists.
These cards are unique because "The Eye of Timaeus" can fuse with other monsters to create new monsters, while "The Fang of Critias", when activated, can fuse with Trap Cards to create monsters that possess similar effects to the Trap it fuses with.
Whilst their color (blueish) indicates them to be Ritual Monsters, they in fact do not belong to any regular card type. This can be backed by the fact that they are never played face-down, hence not Trap Cards, they can be played whilst "Spell Canceller" is on the field, hence not Spell Cards and they can be played during the turn a monster has been Normal Summoned (despite they are sometimes "Summoned" or "Activated") in the Monster Card Zone.
Presumably, since they are played in the Monster Card Zones, they count towards the 5 monster limit of a Duelist. However, Yami Yugi was shown to place "The Eye of Timaeus" in the Spell & Trap Card Zone during his Duel with Weevil Underwood.
As stated above, the Legendary Dragons can combine with cards to create new forms of those cards.
- "The Eye of Timaeus" can fuse with other monsters to become new Fusion Monsters. In the anime, Yami Yugi fused this card with "Dark Magician" and "Dark Magician Girl" to form the respective Fusion Monsters : "Amulet Dragon" and "Dark Magician Girl the Dragon Knight".
- "The Claw of Hermos" can fuse with other monsters to form new Equip Spells. In the anime, Joey fused this card with "Time Wizard", "Red-Eyes Black Dragon", "Rocket Warrior" and "Big Bang Blow" to make the respective Equip Spells : "Time Magic Hammer", "Red-Eyes Black Dragon Sword", "Rocket Hermos Cannon" and "Big Bang Dragon Blow". In addition, Yami Yugi also fused "Queen's Knight" with this card to form "Goddess Bow".
- "The Fang of Critias" can fuse with Traps to form new Fusion Monsters. In the anime, Seto Kaiba fused this card with "Crush Card Virus", "Tyrant Wing" and "Mirror Force" to form the respective Fusion Monsters : "Doom Virus Dragon", "Tyrant Burst Dragon" and "Mirror Force Dragon".
All three cards can also be chained together with "Legend of Heart" to Special Summon the three Legendary Knights; "Legendary Knight Timaeus", "Legendary Knight Hermos" and "Legendary Knight Critias". These three Knights, under the right conditions (Infinite Attack Needed), can also be used to Fusion Summon, "Knight of Destiny".