Tales of Phantasia
As the only named magitech weapon in Tales of Phantasia, it is the result of magic research in Midgards during the past era, developed by its scientists for military purposes. During the original iteration of the Valhalla War against Dhaos, Midgards manages to fire two shots against the invading forces, fully depleting mana on Aselia and killing the Great Tree Yggdrasill. This results in the death of all Spirits and their associated magic, and eventually Dhaos's defeat as he escapes into the future, finding himself caught in a trap and sealed ten years before the start of the story.
When Cress Albane and Mint Adenade travel back in time to seek help in defeating the Dhaos of their time period, they come across Midgards and its technology. Their presence causes the end the Valhalla War many years earlier, resulting in the Mana Cannon being fired only once before being shut down by unexplained means, thereby preserving the Yggdrasill and its Spirits. However, the shot still causes a sharp drain in the world's mana. Martel, the Spirit of the Great Tree, even states outright that mana is fading.
The scene is further expanded upon in Tales of Phantasia: The Animation where the effects of the mana depletion are more tangible. Claus F. Lester summons the Water Spirit Undine to ward off some of Dhaos's invading forces, but the magitech weapon's shot causes Undine excruciating pain, and she vanishes without mana to support her. Rather than simply shut down, the weapon violently explodes in a blast of energy and causes immense devastation to the city. Mint is even forced to defend herself with a barrier when the energy swamps her.
Tales of Phantasia: Narikiri Dungeon
In the sequel to Tales of Phantasia, it is revealed that a similar weapon, also known as a Mana Cannon, resulted in the complete destruction of Derris-Kharlan. The weapon was used during a conflict between two small countries on Derris-Kharlan, known as Palace Gudora and Palace Sedamyu. Dhaos was the leader of a third, larger country which held its neutrality. During this conflict, research into the Mana Cannon was performed by Palace Gudora with Meltia Bandi as the head of development. When the country came close to defeat, Dios Bandi commandeered the weapon under orders from his dying father, General Bandi, firing it toward Palace Sedamyu and destroying it completely.
While the weapon was fired only once, all life on the planet relies completely on the mana their World Tree provides, and the weapon managed to kill the Tree itself, dooming the entire world. Dhaos, realizing that his inaction had led to this disaster, was sent to Aselia in order to acquire a mana seed to aid in reviving the Tree of his world. Upon arriving on Aselia, he discovered that the same kind of technology was already being researched by the local population, and he set out on his self-given task to destroy that technology and secure the safety of the World Tree of Aselia, leading into the events of Tales of Phantasia.
Tales of Symphonia
In Tales of Symphonia, the Mana Cannon is a weapon of mass destruction which was created during the Kharlan War, known as Thor's Hammer during that time. In the game, it is used only once in order to stop the germination of the rampaging Giant Kharlan Tree. Rodyle, one of the Desian Grand Cardinals, is widely active in the construction of the Mana Cannon. The builders are the captives of the Remote Island Human Ranch. It resembles a white tower with a green dome suported by caryatid-type columns.
The project is one of Rodyle's schemes to control Cruxis. He talks Kvar and Magnius into the project but keeps it a secret from everyone else. Kvar gets data from the Angelus Project, and Magnius comes up with the funds for the operation, while he himself obtains the blueprints for his construction. Kvar and Magnius never learn that Rodyle had planned to use the cannon to destroy the Tower of Salvation and build his own empire. Although this is his plan, Rodyle does not know that he was being manipulated by the Renegades into build the Mana Cannon in order to germinate the Great Seed.
Lloyd Irving and his party learn about project through Pietro but do not find out more about the project until later, when they meet the Renegades at the ruined Palmacosta Human Ranch. The Renegades explain their plan to use the Mana Cannon to revive the Giant Tree. The party joins forces with them in order to defeat Rodyle at the Remote Island Human Ranch. Before he dies, Rodyle triggers the self-destruct sequence to destroy both the Ranch and the weapon. If the Cannon is destroyed, the Renegades would be unable to germinate the Great Seed, and all would be lost. Just in time, Botta and his team stop the self-destruct sequence by sacrificing themselves.
The Cannon is used later after the rampaging Great Seed, destroying Palmacosta and the Sylvarant seals in the process. The Renegades know that the only way to stop the Great Seed is to shoot mana at it from the Mana Cannon so that it can no longer be germinated. As the Great Seed is filled with the mana of Sylvaranti Summon Spirits, Sheena uses the Tethe'allan Summon Spirits in the Cannon to balance and calm the mana of the Great Seed.
Tales of Symphonia: Dawn of the New World
Although the weapon itself is not shown in the game, the Vanguard intends to use the Mana Cannon to destroy the Church of Martel, the Tethe'allan Royal Family, and the main cities in Tethe'alla, especially Meltokio. To do this, they need to recover Ratatosk's core, which rests in Marta Lualdi's forehead, as well as the Centurions' cores.