Catelyn goes to pray in the village sept, and figures out that the Lannisters must have tried to kill Bran because he knew about the incest. Then Ser Robar fetches her back to Renly's tent where they have a chat about all that. But it makes no difference and Renly goes off to battle.
Except this is GRRM and what should obviously happen next never happens. Instead a shadow ghost slices Renly's throat mid-jest. No-one saw that one coming, though perhaps Stannis' glowing sword was a clue. He dies. Brienne is hysterical. Angry men come and attack her but Catelyn manages to convince Robar that it wasn't Brienne but a ghost who murdered Renly. No-one else is going to believe that story, though, so Catelyn and Brienne scarper. The camp is in chaos and now there won't be a battle because there is no leader. Stannis has won. Presumably his Red Priestess is somehow behind this sorcery.
So much for sorcery being the sauce used to hide the flavour of incompetence. This is real magic, seen by a reliable witness. It can't be something that happens every day because the maesters are all convinced that if there ever was magic then its power faded long ago. Something is changing in the world: the opposite of Tolkien's magic fading from Middle-earth. Maybe elves have arrived; maybe it's that comet and/or those dragons. Or maybe it's whatever is going on beyond the wall.
No doubt we will spend many frustrating chapters with nobody believing Catelyn or Brienne and everybody continuing to refuse to believe in magic despite more and more weird stuff happening. Or maybe GRRM will surprise us yet again.
