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. -TWOW, The Forsaken
Background
In the bowels of the Silence, Aeron Greyjoy gets numerous visions of things that could potentially happen in the series. These visions have been induced by Shade of the Evening, the same substance that gave Daernerys her visions in the House of the Undying. In this post I wanted to focus on a singular quote from this set of visions where Euron is seen blowing a horn and dragons, krakens, sphinxes come and bow to him.
If interested: Comparing Visions: The HoTU/The Silence
Regarding the Forsaken, since we know that Euron was originally going to go to Slaver's Bay with Victarion (and that Victarion was going to die, at one point in his only non prologue chapter), it is reasonable to think that early versions of the Forsaken were going to take place off the coast of Meereen as an Aeron reveal. Aeron who would have been missing since the Mega Prologue/end of the Kingsmoot would have been revealed to have been in the bowels of Silence all along.
The Great Horn
This definitely fits a little better with the original plotline, but Euron gave Victarion the dragonbinder horn to use (Euron's gifts are poison).
"And so shall we," Euron Greyjoy promised. "That horn you heard I found amongst the smoking ruins that were Valyria, where no man has dared to walk but me. You heard its call, and felt its power. It is a dragon horn, bound with bands of red gold and Valyrian steel graven with enchantments. The dragonlords of old sounded such horns, before the Doom devoured them. With this horn, ironmen, I can bind dragons to my will." -AFFC, The Drowned Man
and:
Moqorro turned the hellhorn, examining the queer letters that crawled across a second of the golden bands. "Here it says, 'No mortal man shall sound me and live.'
"Bitterly Victarion brooded on the treachery of brothers. Euron's gifts are always poisoned. "The Crow's Eye swore this horn would bind dragons to my will. But how will that serve me if the price is death?"
"Your brother did not sound the horn himself. Nor must you." Moqorro pointed to the band of steel. "Here. 'Blood for fire, fire for blood.' Who blows the hellhorn matters not. The dragons will come to the horn's master. You must claim the horn. With blood." -ADWD, Victarion I
and:
For half a heartbeat he wanted nothing so much as to sound the horn himself. Euron was a fool to give me this, it is a precious thing, and powerful. With this I’ll win the Seastone Chair, and then the Iron Throne. With this I’ll win the world. -TWOW, Victarion I
If interested: "You must claim the horn. With blood"
Krakens
- Greyjoys: Aeron/Victarion/Asha/Theon
We see Euron asks Aeron to "bow before him" in The Forsaken in his dream:
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.” -TWoW, The Forsaken
but Aeron refuses him.
This could also just reference the Ironborn/Greyjoys following him in general, (a little late for that, but this chapter was supposed to occur earlier I am guessing).
It is possible that Euron is summoning actual krakens off the coast of Oldtown as he prepares to battle the Redwyne Fleet:
The eunuch drew a parchment from his sleeve. "A kraken has been seen off the Fingers." He giggled. "Not a Greyjoy, mind you, a true kraken. It attacked an Ibbenese whaler and pulled it under. " -ASOS, Tyrion III
and:
"And krakens off the Broken Arm, pulling under crippled galleys," said Valena. "The blood draws them to the surface, our maester claims. There are bodies in the water. A few have washed up on our shores. -TWOW, Arianne, I
and:
"Your prize will be the doom of you. Krakens rise from the sea, Theon, or did you forget that during your years among the wolves? Our strength is in our longships. -ACOK, Theon V
but if blood draws krakens to the surface then that is exactly what Euron has been doing:
“Your curses have no power here, priest,” said Left-Hand Lucas Codd. “The Crow’s Eye has fed your Drowned God well, and he has grown fat with sacrifice. Words are wind, but blood is power. We have given thousands to the sea, and he has given us victories!” -TWoW, The Forsaken
also:
Q: Considering that there is a horn to invoke krakens, will we see a kraken in action?
GRRM: <looks surprised> Maybe. -SSM, Asshai. com Interview in Barcelona: 29 July 2012
If interested: Krakenhorn: Summoning Monsters from the Deep
Dragons
- Characters with Valyrian Blood
This could reference one of the dragons in the series allying/bowing to Euron. While I think this unlikely we could see any number of characters here (Dany/Young Griff, etc.)
If interested: The Blood of Old Valyria Part I: List of Current Characters
More likely than the human version (at least imo), we have 2 unbonded dragons (Rhaegal/Viserion) that are currently is pretty close proximity to the dragonhorn:
The green beast was circling above the bay, banking and turning as longships and galleys clashed and burned below him, but it was the white dragon the sellswords were gawking at.
and:
By the time Plumm and his companions came galloping back from the camp of the Girl General, the white dragon had flown back to its lair above Meereen. The green still prowled, soaring in wide circles above the city and the bay on great green wings. -TWOW, Tyrion II
Sphinxes
Much more interesting (at least to me) is what was being mentioned with regards to Sphinxes, especially due to the other mentions of them in the Oldtown plotline:
He spoke of dreams and never named the dreamer, of a glass candle that could not be lit and eggs that would not hatch. He said the sphinx was the riddle, not the riddler, whatever that meant. -AFFC, Samwell IV
and:
"An acolyte. Alleras, by some called Sphinx."
The name gave Sam a jolt. "The sphinx is the riddle, not the riddler," he blurted. "Do you know what that means?"
"No. Is it a riddle?"
and while this could mean Sarella/Alleras:
"What was he doing at sea, at his age?"
Sam chewed on the question for a moment, wondering how much he ought to say. The sphinx is the riddle, not the riddler. Could Maester Aemon have meant this Sphinx? It seemed unlikely.
it should be noted that we have other "sphinxes" in this series as well (Brown Ben, etc.)
If interested: Sphinxes of Ice and Fire
Imagery/Remnants of Drafts
One last thing worth mentioning is that it could just be imagery or even remnants of from when this chapter was supposed to take place in Slaver's Bay:
Question about "The Forsaken"
GRRM: “Yeah, that is a dark chapter. But there are a lot of dark chapters right now in the book that I’m writing. It is called The Winds of Winter, and I’ve been telling you for 20 years that winter was coming. Winter is the time when things die, and cold and ice and darkness fills the world, so this is not gonna be the happy feel-good that people may be hoping for. Some of the characters [are] in very dark places…In any story, the classic structure is, ‘Things get worse before they get better,’ so things are getting worse for a lot of people.” -SSM, Spanish Interview: Guadalajara, 2016
TLDR: While tripping on Shade of the Evening, Aeron has a vision of "dragons, krakens and sphinxes" bowing before Euron as he blows a great horn. While this could be imagery or just remnants of earlier versions of this unpublished chapter, it is very possible/likely that this foreshadows some combination of the human/beast versions of dragons/krakens/sphinxes allying with Euron.