So well... what the hell is... that? I think it's quite safe to use the word 'sarcophagus', but apart from that... nothing is to be taken for sure!
The banners there for sure tell a story, but we cannot make anything solely out of that (aren't you annoyed by those stories in which the characters see old paintings or whatever and decypher them all in a glimpse and understand the whole history of a disappeared civilization?)... Besides, things tangle up a little bit more when Brent sees that strange glyph and gets a sudden vision of the beast in his dreams.
But once again, all those matters will have to wait, as we have a pressing issue right now such as, well, Orks!