Okay, so here's my theory on Callandor. It obviously plays a vital role at some stage in the future, but no one knows what.

Lews Therin said that the problem with his strike against the Dark One was that something had to touch the Dark One to seal him in. They used saidin, and this allowed the taint to occur when the Dark One retaliated.

What I think will happen is that Rand will touch the Dark One using the True Power (through Moridin), and then seal the patch perfectly using saidin and saidar. These are the three elements : saidin, saidar, the True Power - and the three will become one. The counterstroke this time will strike the True Power, and not harm the True Source like last time.

So why is Callandor unique? RJ said that angreal do not allow a channeler to draw more of the True Power like they do with the One Power. Therefore Rand is limited to his unaided strength with the True Power. Callandor might be unique because it magnifies the taint. No other device does this - not even when Rand channels a million times as much saidin through the Choedan Kal does he get affected as much as when he uses the Crystal Sword. The magnification of the taint may be a synonym for the sword's ability to open Rand to massive amounts of the True Power. Without the buffers, it makes this device unique.

Therefore, several things have improved for Rand since Lews Therin's attempt...

1. Rand can pull the True Power into him through Moridin, and this is the reason for the link. The Dark One will not cut his Nae'blis off from the TP until it is too late.

2. The circle of three will protect Rand from the mental effects of the sword, if there are still any to be had from clean saidin.

3. The use of saidin and saidar will allow for a perfect patch. They've always worked best together.

4. The sword is the only sa'angreal in existence that uses a flaw to allow for a gain i.e. to draw huge amounts of saidin or the True Power.

5. Using all three powers he will seal the Bore.