Star Striders

During the 21st century, what looked to be a long-period comet reentered the Solar System. Early observations suggested that it was not quite what it appeared to be, but then, oddities were commonplace. Probes were launched to investigate the interloper, but little attention was paid during the intervening months.

The excited chatter of the artificial sapience guiding the first probe caught Earth's attention - the first high-resolution images made it clear that this object was not natural. It was immense, however, some three hundred kilometers in diameter. It was once a much larger craft, but time - and war, from the look of it - had taken its toll.

Almost overnight, Earth's entire social, political and religious dynamic was transformed.

Compared to the technology they were investigating, humanity's manned investigation was laughably slow, and primitive. The owners of the relic they were exploring were nearly a billion years dead, however, and most of the vessel's defenses had long since corroded into impotence.

That is not to say that there were not casualties. Some would say that the entirety of the human race was a casualty.