Dialga is a legendary pokemon who is a dragon/steel type pokemon and is the mascot for Pokemon Diamond. It alongside Palkia and Giratina form the legendary creation trio with Dialga representing time. Its signature move is called the Roar of Time.
Dialga was created by Arceus alongside Palkia and Giratina, to govern the flow of time in the pokemon universe. According to the Shinnoh myths, it was considered to be the ancient deity of time because when it was born time began to move. The story of the legendary pokemon of Sinnoh was passed down during each generation in the region who revere it as a deity.
Dialga made its first appearance in the Diamond & Pearl series movies. It first appeared in the Pokemon movie Rise of Darkrai, where it fought Palkia in their own dimensions when the dimensions began to distort and it along with Palkia considered each other an intruder and began a battle with one another. Eventually, both Dialga and Palkia unleashed their signature moves; Roar of time and Spacial Rend in which both started affecting the real world in which Alamos town ended up being in the unknown dimension and from there continued to clash in which their fight was destroying Alamos town. Dialga and Palkia eventually fired their attacks at Darkrai who was trying to hold back the two in order to stop the fighting. As the two continued to fight, the two legendaries charged up both of their attacks which threatened to destroy the dimension completely if it connects. Just then Ash and Dawn with the help of their pokemon managed to fix the space-time tower and reality is restored as it continues to shine and the Oracion song is played. As the tower shines both the two combatants calmed down and returned to their respective dimensions.
Dialga made another appearance in the second Diamond & Pearl series movie called Pokemon: Giratina and the Sky Warrior. In the second movie, Dialga was attacked by Giratina, who was angered that its home: the distortion world is in ruins because of Dialga and Palkia's battle and thus began another confrontation against its fellow legendary. Giratina dragged Dialga to the Distortion World in which the two began to battle. As it was ensnared by Giratina, Dialga tried to escape its clutches to no avail until Shaymin managed to free it. After being freed, Dialga ensnared Giratina in a time-loop where it made its escape. While it did not do much in the movie, Dialga at the end was still being attacked by Giratina.
Dialga's final movie appearance was in the third and final movie of the Diamond & Pearl series called Pokemon: Arceus and the Jewel of Life.