Simple theory which, after reading ToM seems even more likely. The key to Rand's victory over the DO lies in the object Matt brought into the world in his first brush with his "luck". In tEotW Matt isn't sure why he brought the dagger out of Shadar Logoth, just that he was holding it when everyone ran. Now we haven't seen much of this thing since PF went wandering, but we know it is perhaps even more powerful since the cleansing.
With SL being used to in the cleansing in the evil vs. EVIL capacity, I deem it likely that somehow the dagger will provide a big ole key in that last battle.
Personally, I hope it's not in PF's hands, as that would smack a bit too much of a LotR rip off, as so many have pointed out.

Thoughts?