YO MY FAM. SIT DOWN YOU ARE IN FOR THE RIDE OF YOUR LIFE. YOUR VIEW OF WESTEROS WILL BE CHANGED FOREVER.

I do not use the gif above ironically, unlike most times when it is used. This is a pretty decent representation of the theory.

The concept of the Eldritch Apocalypse theory is that while we’ve been dealing with two main forces in Planetos/Terros (ice and fire) there is a third supernatural force that is going to come around and bring about the end of Westeros as we know it. This doom will come at a pivotal point in the story, probably after Dany is in Westeros already. It will, among other things, probably lead to the fall of the wall.

How, you ask? I’m not going to do justice with an explanation, so feel free to skip to my last paragraph where I link to the original poster and his tag for things, plus some other fun resources.

But essentially, Euron Greyjoy (our friendly caricature of a pirate, eyepatch and all) is a former pupil of the Three Eyed Raven/Brynden Rivers. He saw into the deep dark heart of winter, realized the Others were coming, and then peaced out of his responsibilities. Then, he went on his quest around the world and Learned Some Things, especially while he was in Valyria.

His current mission is to go to Old Town (where a certain maester-in-training is hanging out- this will be important later!). In Old Town, Euron will Do Things that will Wake Something. This Something is involved with some kind of a dark “faerie realm” type place, tied to the hinges of the world that all are denoted with dark slimy stone. 

What is that place? The Tower! Where there also happens to be a crazy Hightower maid who potentially knows some magic!

And in Old Town we have Samwell Tarly, who is in possession of a horn from Beyond the Wall. Do you remember the horn? The horn was hidden with the stash of dragonglass that Jon found. Jon gave Sam this horn. The speculation is that it is the true Horn of Joramun. 

Our friendly pirate gets ahold of this horn (somehow?) and blows it (probably on the tower). He brings down the wall and brings about the end of the world by calling up creatures of the deep from the faerie realm, bringing them through some kind of portal in the shiny stone, and turning the world to hell.

And all this (probably) before he gets a dragon. (There’s another theory that google didn’t give me a good link for, but Euron did Something to the Dragonbinder horn that Victarion has; Victarion plans on having someone else blow it; hastily blows it himself; finds out Euron tied the horn to himself already; dies of the blow).

The wonderful @poorquentyn​ is the person behind nearly every facet of this theory, as far as I am aware. I’ve seen other people speculate a little bit about it, like @joannalannister​‘s answer to my ask here (and somewhat, but less so, in the original Sansa post I reference). A lot of this rides on your knowledge/understanding of the books so you have been warned. You can find his tag for everything here and also a few other posts on @asoiafuniversity​ here.

When he laughed his face sloughed off and the priest saw that it was not Urri but Euron, the smiling eye hidden. He showed the world his blood eye now, dark and terrible. Clad head to heel in scale as dark as onyx, he sat upon a mound of blackened skulls as dwarfs capered round his feet and a forest burned behind him.  

“The bleeding star bespoke the end,” he said to Aeron. “These are the last days, when the world shall be broken and remade. A new god shall be born from the graves and charnel pits.”

Then Euron lifted a great horn to his lips and blew, and dragons and krakens and sphinxes came at his command and bowed before him. “Kneel, brother,” the Crow’s Eye commanded. “I am your king, I am your god. Worship me, and I will raise you up to be my priest.”

“Never. No godless man may sit the Seastone Chair!”

“Why would I want that hard black rock? Brother, look again and see where I am seated.”

Aeron Damphair looked. The mound of skulls was gone. Now it was metal underneath the Crow’s Eye: a great, tall, twisted seat of razor sharp iron, barbs and blades and broken swords, all dripping blood.

Impaled upon the longer spikes were the bodies of the gods. The Maiden was there and the Father and the Mother, the Warrior and Crone and Smith…even the Stranger. They hung side by side with all manner of queer foreign gods: the Great Shepherd and the Black Goat, three-headed Trios and the Pale Child Bakkalon, the Lord of Light and the butterfly god of Naath.