The Legendary Dragons are 3 ancient 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.
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.
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 Spell Cards, 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.
As stated above, the Legendary Dragons can combine with other cards to make new forms.
- "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".