If I remember correct, there are three weapons linked to Arthur. A sword, spear and a dagger. If the Knights have the sword, and the athame is the dagger, so where is the spear?
It also occurs to me that there might be some (nasty) possibilities in combination of more ordinary technology and/or magic to deal with immortals. IIRC, Bob told Harry something to the effect that you could kill an immortal (Maeve in that specific case) and mulch the corpse, and all you'd do (except on Halloween) would be to delay her return.
So suppose someone wanted to keep an immortal, say Mab for an example, down. Assuming he managed to kill her in the first place, you could mulch the corpse, then put a tracer spell on the remains, or otherwise attach some kind of magic to tell you what the mantle was doing, and when it starts to bring her back...mulch her again before she can recover. Or things along those lines. If you really had it in for her, you could even make sure each successive death was a whole new agony, sort of like what she did to Lloyd.
No wonder Mab wants Bob's knowledge out of circulation...Bob told Harry that one of the early Council Merlins apparently knew about the Halloween effect, because he originated the custom of masks on Halloween as a protective ritual. I wonder what ever happened to that Merlin, and if the Senior Council people might still know the truth?