The Valhalla War is a major conflict that occurs during the events of Tales of Phantasia.
The Valhalla War began in 4202 of the Aselian calendar, during the past timeline of the game's story. The conflict takes place between the Kingdom of Midgards and Dhaos the Demon King's army, with the battle itself primarily taking place on the plains of Valhalla. While Midgards would normally have the support of the Kingdom of Alvanista, Dhaos intervenes and manages to influence the royal family of Alvanista. Dhaos's reasons for initiating the war stem from his desire for a healthy World Tree, and Midgards is known for its extensive research using magitechnology, which consumes masses of mana that gradually kill the World Tree. Dhaos himself requires a mana seed for the sake of his homeworld and realizes that such is impossible with the world's current overproduction of magitech, particularly Midgards.
Dhaos first attempted to take a more peaceful approach by addressing Reisen, the man responsible for Midgards's research in magitechnology, directly, but Reisen refused. Being left with no other choice, Dhaos gathered demons and formed an army to declare war on Midgards and halt its dangerous methods. During the war, however, Reisen employed the recently-developed Mana Cannon, which unleashed an enormous force of magic that destroyed a large portion of Dhaos's army, but it consumed so much mana in the process that Martel herself, the Spirit of the World Tree, felt its mana disappear. A second attack was declared but failed due to a defect.