Mecha anime series by Sunrise that aired in 1999. It shares the same setting as Yuusha Ou GaoGaiGar, though it has a much darker and more serious tone than it's counterpart. Despite being a spinoff of GaoGaiGar, it is often not considered part of the Yuusha series on it's own.
The series was followed up in Hakaiou: GaoGaiGar vs. Betterman.