Deathtoll (who is also known as Dark Gaia) is the main antagonist of Soul Blazer and is the ruler of all evil, comparable to Satan in other media - prior to the beginning of the game a misguided King Magridd had one of his subjects (Dr. Leo) create a machine by which to summon Deathtoll, causing the archdemon to be unleashed upon the world: upon being freed Deathtoll offered King Magridd a gold piece for every soul in his kingdom and under the counsel of his power-hungry wife, Queen Magridd, the King agreed but was betrayed by Deathtoll and became imprisoned himself.
However after these events the chief deity of the Soul Blazer universe, known as The Master (again, with the alternate title of Gaia), decided to restore the world and tasked the game's protagonist with reviving the world via liberating all the sould Deathtoll had imprisoned.
In order to try and stop the hero from succeeding Deathtoll unleashed many minor demons and monsters via portals across the world as well as tasking higher-ranking demons to try and defeat the hero.