Neat story but what a crock of shit... or maybe it's just some clever writing from George, so let's try and decipher this sphinx's riddle. I'm here to propose my theory on lightbringer and the cause of the long night.
To get started I am not buying the whole lightbringer being some super sword able to end the long night, after all it is just a sword even if it does catch fire, dragons seem like they would be a much better weapon for the war. In fact I think lightbringer will do the exact opposite of ending the long night and cause it. My theory is that lightbringer isn't a sword at all (well it is but I'll get to that later) but actually a falling star or in other words a Red Comet. That Red Comet appeared in the sky 3 times before it brought on the long night, and on its third rotation plunged into the teets of Nissa Nissa causing her scream to crack the moon...wait that doesn't make sense, I change my mind, the moon or should I say moons at the time are Nissa and Nissa. After all moon is Goddess Khaleesi.
First let's start with the Red comet, it is called many things but 3 main things stick out to me. Varys says the people of kings landing call it the red messenger (as do the people of the riverlands) and he says furthermore that preachers and prophets say it foretells of Doom and Destruction. Old Nan says she can smell it and it means Dragons. Gendry believes it looks like a Red Sword and Arya thinks it looks like Ned's sword Ice covered in blood. I believe it's all 3 of these things but for each rotation (forging) of the red comet. The first comet to appear we learn from Maester Aemon, which appears before AGoT and it is the one Rhaegar believes to symobolize The Prince that was promised. He may be right but if he is I don't think that prince is of his seed, I think the first comet was the Red Messenger which woke the Others, this is the first "forging" of Lightbringer. The second cycle of the Red Comet we see towards the end of AGoT and that one I believe Old Nan is correct and it symbolizes dragons...but wait the comet was in the sky before Danaerys dragons were born. Now I don't want to lose some of you so let's just say it means dragons (I think it woke a shadow dragon in the first keep but I'll save that for another post), this was the second "forging" of lightbringer. My predictions for TWOW is that we will see the Red Comet again and with it will come the long night and Azor Ahai, and this "forging" will be that of Lightbringer the Red Sword of heroes.
Now the moon is mentioned a lot in ASOIAF from the Others resembling moonlight, to the Dothraki believing the moon is a Goddess, to Nissa Nissa cracking the moon, to the Qartheen saying dragons came from the moon. While I don't think dragons physically came from the moon, I think they appeared because of it or rather lack of it. We have 3 accounts of a moon being destroyed/damaged, one from the Nissa Nissa story, one from the Qartheen believing dragons hatched from the moon, and one from Benerro. When Tyrion is in Volantis he spys Benerro and Benerro points to the moon and makes a fist, he then opens his hand as if saying the moon exploded. He then writes out Valyrian glyphs in fire and Tyrion is able to make out 2 of them, Doom and Darkness. It's almost as if he's saying the moon cracking brought on the long night. A quick google search on the effects of our moon exploding made this pretty obvious. If our moon were to crack first off pieces of the moon would rain onto the planet, which I believe would be the oily black stone we see throughout planetos. Second it would likely bring on a new ice age, which I would compare to the Long Night. Third it would alter the seasons, which accounts for why Planetos' seasons are all fucked up. And fourth it would alter the tides causing our oceans to shrink, which is why the Dothraki Sea dissolved and Sea dragons entered the world...wait what?
Now we hear nothing of Valyria before the long night, and that is because I believe until the moon was destroyed the 14 flames and all of Valyria were submerged "under the sea" as patchface would say (I'll do another post on patchfaces connections with Valyria). Once Lightbringer struck Nissa the sea's shrunk and the previously covered volcanoes were exposed, and out burst Dragons. Since Valyria came from the sea you can say the dragons did too, hence the use of Sea Dragons throughout the story, and it would explain why the Qartheen believe dragons came from the moon. These dragons were before the time of Valyrians and dragonlords, new to the world so It's likely they were uncontrolled and wreaking havoc wherever they went, because Fire consumes while Ice preserves. This would also account why there were lots of underground tunnel systems throughout the world and places like casterly rock and Skagosi caves, as we saw with Aegon's attempted conquest of Dorne, dragons can't hurt you underground. I believe a deal was made with the Others because of these dragons and that's where Azor Ahai comes in.
Now according to Old Nan the Others hate every living thing with Hot blood in it's vains, well what has hotter blood than a dragon? I think the CoTF, the First Men, and the Others made a pact, and part of that pact was to rid Westeros of Dragons, and the person to do that was Azor Ahai. The Others gifted a weapon to Azor Ahai to accomplish this, a weapon a lot like their own, pale milkglass alive with moonlight. This weapon was crafted by the Others out of a fallen star and It was named Ice. Yes I'm saying Dawn the ancestral sword of house Dayne is actually the original Ice and the Daynes are the "keeper of the sword" (my guess to what Dayne's words are). The Daynes are holding it because it's basically as far away from the North you can get without leaving Westeros (this part stumps me, but it does make more sense to me that Azor Ahai was a Stark) but it was actually a Stark who was the original wielder. This Stark armed with Ice then went to slay a great dragon who was burning all of Westeros.
It wasn't that the blade was fiery hot, it was the dragons blood that made it fiery hot, and it could possible retained some of the properties of dragonsblood (kind of like the sword of Gryffindor and the basilisk blood). Once he slayed the dragon he carved the horns off and crafted 2 warhorns; Dragonbinder and the horn of Joramun, A horn of fire and a horn of ice. Dragonbinder was of course used to tame the wild dragons pouring out of Valyria, burning the world. I'm not so certain what the horn of Joramun will do, but I do not think it will bring down the wall. In fact i think after Azor Ahai completed the pact, the Others would help Brandon Stark build the wall. It's a wall of Ice and who better to help build a giant ice wall than the most skilled ice mages in the world. I don't think the wall is so much to keep out the Others but to keep out the Dragons, as we know from Queen Alysanne silverwing would not fly past the wall.
To finalize I think the Great battle will finally happen after we see the third comet in TWOW. While it is in the sky Azor Ahai (Jon Snow) well be born amidst salt and smoke. I think there are few options for what Azor Ahai will do, but I do think he will wield his milkbrothers ancestral sword Dawn/Ice. Now if history repeats itself he will slay a dragon maybe Dany, maybe a shadow dragon, maybe drogon, or it could be a metaphor using his "sword" to impregnate Dany. I don't think the latter is likely for I have my own Idea's about Dany and I don't know if Jon's seed can still sow after being reborn. Fair warning I am also submitting this without proof reading so sorry in advance, but I'm too damn tired to go through this all again, I know it's very likely that there are some small additions I forgot to add but I'll post them tomorrow.