Any thoughts on what Kringle meant by “Uriel’s boundary line”?

I'm banging out Dragoncon 2013 right now, just as an FYI to everyone.

It is an interesting theory, Mac being Tam Lin or perhaps Merlin.

But there is a line from CD that makes me wonder if Mac isn't something else entirely. I don't have the book in front of me right now, but Sharkface does call Mac Watcher once or twice. There is a very specific class of being who were referred to as Watchers. The Grigori.

Yea I think this has been mentioned a couple of times - warrants some more research in my opinion.

Currators note:  This OP has some pretty awesome stuff in it, and the topic quickly morphs into finding ties between Tam Lin and Mac ^-Serack

 I'm not sure why it took me so long, but I finally did some research on Tam Lin, the Winter Knight that Mab so fondly remembered.

Dancing with Mab was like dancing with a shadow. She moved so gracefully, so lightly that had my eyes been closed, I might not have been able to tell that she was there at all. I felt lumbering and clumsy beside her, but managed not to trip over my own feet.
      “That was well-​done, wizard,” Mab murmured. “No one has lifted a hand to them that way since the days of Tam Lin.”

Here's the summary of the Tam Lin ballad:

The woods of Carterhaugh are guarded by Tam Lin, a man who demands payment of all maidens who pass through, in the form of a belonging or their virginity. A maiden named Janet travels to Carterhaugh and picks a rose, causing Tam Lin to appear. He questions her presence, to which she relies that Carterhaugh is rightfully hers. She then travels to her fathers house where she exhibits the early signs of pregnancy, much to the concern of the household. She states that her lover is elven, and then returns to Carterhaugh, once again encountering Tam Lin. He reveals he is not elven, but a mortal captured by the queen of Faeries, and that he may be sacrificied to hell as part of the faerie tithe. He then details how she can save him to be her mate, if she will undergo a trial on Halloween night. She must pull him from his horse as the faeries process through the woods, and hold onto him as he is transformed into various beasts, then plunge him into a well when he turns into a brand of fire. When he regains his own naked shape she must cover him with her green mantle and he will be free. She does all of this, much to the anger of the watching Queen of faeries.

I'm trying to pick this apart to see what we can extrapolate to the Dresdenverse here. Some thoughts:

• It seems as though if the Queen decides to sacrifice her knight on Halloween for the tithe and he manages to survive it, the mantle of knight will be discarded. This fits beautifully with Vadderung's statement at the end of CD:
  
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  “It was Halloween, Dresden. You put on a mask for a time. That’s all.” He looked directly at me and said, “Many, many mantles are worn—or discarded—on Halloween night, wizard.”
• Interestingly, Lin was covered by a physical "mantle" in order to be relieved of his knight's mantle. Perhaps the knight mantle is simply being switched for a physical one - this would fit with the balance we see with the Sidhe, though it's a bit of a stretch. Alternatively this could be a metaphor for another magical mantle that is being bestowed, and the story has been distorted through time.
• The tithe is a ceremony that happens every seven years in which mortals are sacrificed to hell, though why this occurs seems to be unclear in the general folklore. Some sources state that it is payment to Satan for something that happened in the past. This could easily be a plot point for a future book.
• It's possible that the well referred to is the Wellspring at the heart of Arctis Tor, which would help to explain exactly how the mantle is discarded - the mantle is kept at the heart of the power of Winter until it is bestowed upon a new knight. The knight is thrown down the well in order to put the mantle in contact with the heart of winter.
• Tam Lin needed a woman to help him throughout the process - perhaps if Harry is to undergo it as well, this would be represented by Murph? Or perhaps this is the woman that Jim decides will be the girl for Harry's future.

It's not airtight, but I do think it's revealing. One big problem is that now that we know the purpose of Winter, Mab, the Knight, and the Sidhe in general, it doesn't seem as though Harry should be discarding the mantle of knight - he needs the power ups in order to fight the outsiders. There are at least three possible solutions to this. The first is, as suggested before, the mantle of Knight is being switched for a different magical mantle that we are currently unaware of (though the GateKeeper mantle is a possibility). This would allow Harry to retain his powerups, or at least be given some new ones for the continuation of the series. The second possible solution is that the tithe ceremony happens at the very end of the BAT, after the outsiders are destroyed. This would be a fantastic opportunity for Harry's love life to be tied up and his ties with winter to be severed, creating a very "happily ever after"-esque ending. The third possibility is that the maiden is Lea; the mantle could be a metaphorical or physical one that she gives to him after the trial. I'm thinking that within it she bestows upon Harry the power that she was given to by Mab when Mab traded for Harry's debt. This would represent the final payment for the debt that Lea had to Maggie senior. It's possible that this would give Harry the power of Winter without the obligations of the Knight.

Is there anything else we can garner from the Tam Lin story?