Symphogear Decade is a Massive Multiplayer Crossover between the world of Symphogear and the author other MMC work, the Ultimate Tokusatsu Chronicle. It was probadly the sanest work written by Lord Bá Đạo ever.
The world of Symphogear Decade is just like the world in Symphogear but having a distorted version of the Ultimate Tokusatsu Chronicle mythos mixing with it own mythos. Due to an interference to the structure of it Space-Time Continuum by Foundation XYZ.It contain many element that didn't exist in the original Symphogear World.
In this world, there are many race of monster and organization that try to change the world into their image. The Noise are just one of the worst dangerous foe the Relic User has to face. And the world are constantly in danger from all the other organization way before the Zwei Wing Concert happen.
Zwei Wing Arc
The story about what happen before and during the Zwei Wing Concert.
Val Verde Arc
After the Zwei Wing Incident, Serizawa Yami has to live a double life, both as a student and Dark Decade as he has set foot in Val Verde to kill civilian. His goal is to use those live he took as a way to proved that he really did love his waifu and not merely considered them to be a trophy or object.
2nd Division Arc
This occur after the Val Verde Arc where he officially join the Second Division.