The player is cast in the role of Sieg Wahrheit, a Knight of the Dark Glyphs, who is able to sacrifice lost souls in order to summon creatures called Chaos Legion from alternate worlds. When Victor Delacroix steals the forbidden Apocrypha of Yzarc, Sieg must track his former friend down and use the Legion to put a stop to his actions.
With certain crest items in his possession, Sieg Wahrheit becomes capable of summoning Chaos Legion. Strategically commanding squads of spiritual soldiers, Sieg and his Legions must blaze a path through the hordes of the Nether World, seeking to stop renegade Knight of Order, Victor Delacroix. The size, power, skills and capabilities of each Legion can be increased using Experience Points gained in battle. The Legions come in seven distinct types, perhaps patterned after the Seven Deadly Sins.
- Guilt - Sword Legion
- Hatred - Power Legion
- Malice - Arrow Legion
- Arrogance - Shield Legion
- Flawed - Claw Legion
- Blasphemy - Bomb Legion
- Thanatos - Ultimate Legion
Tis the darkest Glyph thou barest, it sacrifices stray souls to summon unearthly force, Unearthly force shall obey and serve thee, 'tis named Chaos Legion. --- Superior, Order of St. Overia
The story of Chaos Legion is a gothic opera which begins November 791 A.S. (Anno Satanis). The protagonist, Sieg Wahrheit, is on a quest to find his former friend Victor Delacroix, who has stolen the forbidden book Apocrypha Of Yzarc, and to stop him from releasing the spirit Azrail who will destroy the three planes of existence (the Nether World, the Middle World, and the Celestial World). In the beginning, Delacroix feels that Sieg was responsible for the death of his lover, Siela Riviere, and is intent on bringing Sieg into a pure pandemonium that Delacroix himself has prepared as retribution not only for the sins of Sieg alone, but for the sins of the whole world. Delacroix refers to this armageddon as the "Purification of the World", whereupon he plans to use relentless hordes of hideous monsters to cleanse the world of sinners.
- Sieg Wahrheit
- Arcia Rinslet - is a "Maiden of the Silver", one of several female warriors toting twin silver guns. Alongside her fellow Maidens, she is dispatched by the Intelligence of the Order to purify towns of demons that have been unleashed by Delacroix. However, Intelligence was unaware that Arcia bears a vendetta against Delacroix, who recently slew her only brother. Alongside Sieg, she pursues him to avenge her family. Arcia is playable if the game is beaten on normal difficulty.
- Victor Delacroix - once anticipated by the Order to be the next Lord of the Sacred Glyphs, Delacroix became a "fallen angel" in his dogged quest to purify the world by collecting the Three Sacred Glyphs. His personality change and migration to the dark side seemed to be brought on by the death of his sister, Siela. The Order he once served and thought so highly of him is now his enemy, especially after he murders a leader of the Order and steals the Apocrypha of Yzarc. Eventually, Sieg confronts him...and they face the tragedy of the past that stands between them.
- Siela Riviere - was a Healer and servant of the Order. She was the only one Sieg and Delacroix both could trust. However, she was killed in a tragic accident three years ago. She is only seen in "flashback" sequences during the course of the game, and at the end, appears as the final boss.
- Azrail - one of the two main antagonists in the game, the other being Delacroix.
- Yzarc, from the "Apocrypha Of Yzarc" book, is "crazy" spelled backwards.
- The name "Thanatos" is derived from a minor Greek mythological figure by the same name. He was the personification of death.