I've been obsessing over trying to figure out the specifics of an event I only vaguely remember reading about in a core rulebook for... possibly 5th? Or may have been 4th. I no longer have access to said rulebook so my vague memory is all I have to go on. Maybe somebody here has a collection and can scan the timeline.
Note: It may be multiple events combined by the vagaries of time and my strange brain, and it may not even necessarily be an event on a timeline. However, this is how my brain remembers it, so sorry if it has some false details.
Some mysterious force is either unleashed on the galaxy by a strange box-like artifact, or is eventually defeated by using a strange box-like artifact. Said box-like artifact may be moon/planet-sized? The entire Imperium/galaxy is at stake during the conflict, and maybe Xenos or even Chaos forces joined with the Imperium to fight this threat off. I keep thinking it's Enslavers, but Lexicanum doesn't refer to any galaxy wide events regarding them except for the "Enslaver Plague" (which they died off on their own, and was before mankind ever arose).
I did not get the feeling, at the time that I read this, that this force was related to the Warp in any way, which made it seem even cooler.
Does this ring any bells to anybody?