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I've read (on Wikipedia) that the Hive Fleets often come from the Eastern Fringe and drive towards Terra.

Is it possible that there's some secret Tyranid artefact on Terra?

I remember reading somewhere that the Tyranids are drawn to the Astronomican. I want to say it was in a Tyranids codex but I can't remember.

The Tyranids are being drawn to the psychic beacon of the astronomican like moths to the light.

Spoiler alert: The Emprah is just one super-genestealer.