Digimon Frontier, the fourth Digimon series, which began airing on April 7, 2002, radically departs from the previous three by focusing on a new and very different kind of evolution, Spirit Evolution, in which the human characters use their D-Tectors (this series Digivice) to transform themselves into special Digimon called Legendary Warriors. After receiving unusual phone messages, the five main characters go to a subway station and take a train to the Digital World. They end up fighting Cherubimon and his group of Legendary Warrior servants, hoping to foil his effort to dominate the Digital World. Later on, they face an even greater challenge as they try to stop the Royal Knights-Dynasmon and Crusadermon-from destroying the Digital World and using its data to revive the original ruler of the Digital World: the tyrannical Lucemon. In general, Frontier has a much lighter tone than that of Tamers, reverting back to the style of Adventure and Adventure 02.