So the fluff from the Traitor's hate book has been summarized and it appears that the story surrounding the conflict between Chaos and the Blood Angels is that on that planet there is a pre-human artifact that is capable of "caging" a warp being. Chaos wants to unlock it with the "Banshee Stone" or something.

Rampant speculation time. What's in the cage?

I've got 100 oomlats on it being the Chaos space rat god.

Whats your guess as to whats in the cage?