About a million years ago the galaxy united to put a Weapon down. It succeeded, barely. The Ship was crippled and buried on the dark side of the Moon, under a million years of regolith. One escape pod reached Earth and sealed itself in a cave, waiting for a Lord Captain it could not yet name.
The two halves waited through geological time. Earth went on being Earth. What that battle left in the system is wreckage, scavengers picking through it, and one ship that waited out the whole million years.
The record that survives is the burial. A crippled Weapon under the Moon. A pod sealed in a cave on Earth. The wreckage of that fight, still in the system.