Background setting (2125): About 70 years after World War III (nuclear war over resources). The earth has become a radioactive wasteland, the ecosystem has been severely degraded, and the survivors have huddled in a few huge underground city networks called "Arks". The social structure is ruled by the militarized "United Human Defense Forces" (UDF). Although technology has been partially restored due to the excavation of "relics", overall resources are scarce and life is depressing and hopeless.
The main source of conflict: The alien "Reaper" fleet - unknown origin, unknown motives, terrifying image (similar to interstellar necrons or mechanical/biological hybrids). They conduct a "harvest" approximately every 50 years, indiscriminately destroying ecological settlements on the surface, but strangely avoid the main "ark". The UDF firmly believe that they are predators/exterminators.
Core setting - Project Valkyrie: In the ruins after the war, humans discovered several highly damaged giant humanoid weapons (20-30 meters high). Their internal structure far exceeded human technology, and the driving core was an unknown crystal (Sol Crystal) called "Sol". Through painstaking reverse engineering, UDF restarted seven of them (named after the Valkyries in Norse mythology: Brünnhilde, Sigrun, Gerndal, etc.) And developed a neural connection operating system, driven by the "Chosen One" (Einherjar) who has been strictly screened and mentally strengthened. They are the patron saints, mobile fortresses and spiritual symbols of mankind.
