Okay, I just finished Peace Talks and I haven't gotten to Battle Grounds yet so please kindly zip your howling screamer if that book answers this, but I have a theory on the British Prisoner.

I remember in Skin Game there is a prisoner in Demon reach with a British accent that sounded tired more than anything, and said he was "someone who deserved to be here". Later, in Peace Talks, King Corb says he remember when Mab rode with the Conqueror, loved Merlin (like the proper Merlin), and wept when he cast her out. Based on this, I have a hunch that Mab is the Arthurian Nimue. If you're unfamiliar with Part of the myth is that Nimue was Merlin's apprentice and an object of his affection, who eventually tricked him after she had learned everything he had to offer, imprisoning him. The nature of his prison varies between tellings, sometimes he sleeps beneath a tree, with the air pulled about him so none can find him, and his voice is heard when the wind blows through the trees of Britain, but in older versions, he's sealed in a tomb or cave, with a rock pulled over the entrance. I wonder if the British prisoner in Demonreach is the original Merlin, and if so, what did he do to warrant his imprisonment?