The five Attendants from Invasion are linked to their respective Dragon Legends by the colors of mana they produce when sacrificed. Each Attendant can be sacrificed to filter one mana into three colored mana, allowing the Dragon Legends to be cast in decks of any color.
That wasn't the first time such a relationship was created. Both Coal Golem and Ball Lightning were printed in The Dark, and the Golem can be sacrificed to turn into , allowing any deck to cast Ball Lightning. Not easily, mind you, but it does make for a nasty surprise.