Inventio Domus Mortis

Our history was of little import when we stepped to the stars. We had put aside our differences, if for no other reason than to provide a united front to whatever we found in the infinite expanse around our homeworld.
We thought we'd either find a galaxy that had cultivated life like ours or nothing at all. We feared the crushing loneliness of a empty galaxy equally as we did the idea of finding hostile alien species simply out for our blood. Somehow, we found the worst of either answer.
Inventio Domus Mortis. Finding the House of Death.
Before we even left Sol, we found our first grave marker. Remnants of a species that had lived within the upper atmosphere of Jupiter. All dead since long before humanity crawled out of the primordial soup, but still their cities floated impossibly, giving no outside sign of their presence. Their bodies still existed, despite the fact that decay should have rendered them to dust long ago.
Yet they were all gathered together. Every city empty but one, which in turn was crammed full to bursting with skeletons. Each one standing on its own, looking towards the city center. Whatever they were looking to left no sign behind on any body or structure.
And each skull was missing completely.
Whatever the implications of such a discovery, we ignored them and forged on. It was our big moment, after all. We could not just stand there and do nothing. It wasn't long after that we found another grave marker made from the corpses of an entire species. This time they were spread across all the planets of a solar system. Their solar system, presumably.
These were scattered, and broken. Petrified by the atmosphere of the planets they lived on. The opposite of the Jupiter Gravesites, it was as if these had died hours before the scout teams had arrived, but the cause could never be identified.
So it went. We crawled from our homes, planting roots in countless worlds that still bore life, alien as it might have been, and with alarming regularity finding the remains of whole civilizations just lying there. We knew we weren't alone, or at least hadn't been, but we also have no idea as to what caused any of those mass death events.
Nor do we know what the criteria is to come under threat by whatever had invited the reaper to their door.