The story of Chaos Legion is based on a light novel of the same name, written by the Japanese author Tow Ubukata. The events begin in the year 791 A.S. (Anno Satanis). The protagonist of the game is a Knight of the Dark Glyphs named Sieg Wahrheit. His best friend, Victor Delacroix, blames him for the death of Siela, Victor's sister and Sieg's lover. Unable to contain his despair, Victor decides to "purify" the sins of the world by unleashing hordes of monsters and finding three sacred glyphs to resurrect the evil spirit Azrail, who would eventually obliterate all existence.

Chaos Legion is an action game with a heavy emphasis on melee combat. The game's stand-out feature is Sieg's ability to summon the so-called Chaos Legions. Named after various emotions (Guilt, Hatred, Malice, etc.), these Legions have different combat styles, strengths, and weaknesses. Sieg himself is weakened once a Legion is being summoned, and has to rely on their powers to survive. The legions can be assigned general combat tactics, but are otherwise controlled by the AI. They can be upgraded during the course of the game, increasing their attack power, duration of their presence, etc.