The Soul Stone is a mysterious object with massive quantities of divine energy that is said to rival against the Deities. Only the Kounat Royal Family who bears the "Geas" imposed upon them can utilize the Soul Stone's power.
The object was created by Baldinar, who is now known today as Ashtaroth. Approximately 600 years ago, he acquired the Klara Libri, a legendary tome that contains the secrets of all of Aernas. Following the book's instructions, he gathered the energies of Aernas' Creators that still lay within the Aernas Hammer and used them to form the Soul Stone, and planned to merge the Soul Stone, the Aernas Hammer, and the Demonic Essence gained from fallen demons in the Demon Wars to gain power equal to the Creator. He used the powers of the completed Aernas Hammer to annihilate the Highlanders, leaving only Sieghart as the sole survivor. However, as Baldinar was not a member of the Kounat Royal Family, who could withstand the 'Geas' imposed on them by the gods, the object grew unstable. By the time he realized it, the weapon was beyond anyone's control and resulted a catastrophic explosion that annihilated Kounat, leaving Mari as the sole survivor. This event also had inadvertently created the Dwarf - Elf War on the Archimedia Continent, as the explosion killed both their kings; both sides blamed each other for the incident and have been fighting each other ever since.
Despite this loss, Ashtaroth was not deterred from his goal. In the present day, after the Grand Chase defeated Thanatos, he stole his Orb of Ascendancy and fled to Archimedia. In Thunder Hammer, Void stole the remaining five orbs that was still in possession of the Grand Chase, and Ashtaroth used them to recreate the Soul Stone.
According to The Awakening, when Mari entered the Relics of Kounat, she fainted. The Soul Stone then somehow contacts her and shows her fragments of Kounat's Destruction: Baldinar, Klara Libri, the Demons, the Wars, and Aernas Hammer. Afterwards, Mari recovers and transforms into the Geas. However, Mari disappears after imploding the Soul Stone, and her earrings fall to the ground as the only remnant of her existence. No one knows what happened to her afterwards. This is proven to be canon in the Wizard's Labyrinth and the Monster Train 301, where Sieghart comments on her disappearance.
The Chase, along with the Existor and the Violet Mages, later engage battle with Ashtaroth, who effortlessly defeats the other two guilds with the Soul Stone, though Sieghart notes that the Soul Stone's powers are incomplete without the key. He then annihilates Harpe Noir with just one attack after he throws himself in front of Ronan, then proceeds to destroy the Grand Chase after weighing them down with gravity magic. Kassias Grandiel arrives in the nick of time to hold off his old friend while the Chase escape to the Monster Train 301.
As the Chase press on to Zeruel, Dahlia and Gilberta reveals that Ashtaroth is re-attempting his original plan, but this time it will not stop with a kingdom's demise: all of Aernas will be consumed in order to recreate it into Ashtaroth's own twisted image.
In Kounat's Collapse, although not mentioned, Ashtaroth had used the Soul Stone to recreate the Aernas Hammer, and draws power from it as he fights the Chase and Kassias Grandiel. However, Grandiel is able to defeat his old friend with the power of the Klara Libri's clone, but suddenly the Hammer warps its key, Mari, to the field. Void arrives immediately after and prevents the Hammer from completing, and Grandiel sacrifices himself to seal himself, Ashtaroth, and the Aernas Hammer in the Wizard's Labyrinth for all eternity.
- Strangely, in-game never mentions the Soul Stone turning into the Aernas Hammer.