I thought I'd offer a different perspective on the "Slayer of Lies" triad. I think it's worth taking another look at.
Glowing like sunset, a red sword was raised in the hand of a blue-eyed king who cast no shadow. A cloth dragon swayed on poles amidst a cheering crowd. From a smoking tower, a great stone beast too wing, breathing fire . . . mother of dragons, slayer of lies . . . (Dany IV, ACoK 48)
The vision is bookeneded and I think it's related to Azor Ahai/the prince that was promised prophecy.
Glowing like sunset, a red sword was raised in the hand of a blue-eyed king who cast no shadow.
I think we all agree that this blue-eyed king is no other than Stannis Baratheon. Melisandre has been adamant that he is Azor Ahai come again. She gave him a shiny sword, is saying that he was reborn on Dragonstone. We don't know why Mel believes this or what she has seen in her flames that has led her to this conclusion. She does however manipulate things to fit the prophecy, such as holding having that "rebirth ceremony" for Stannis in a place of salt and smoke, while the bleeding star is still hanging in the sky.
But Melisandre is missing one thing to "fulfill" the prophecy as she sees it, and that's waking the stone dragon. And I will return to this.
A cloth dragon swayed on poles amidst a cheering crowd.
This is where I peace out of the speculations about Aegon being a fake and a Blackfyre.
We get an explanation from Dany about the meaning of the cloth/mummer's dragon.
"A dead man in the prow of a ship, a blue rose, a banquet of blood . . . what does any of it mean, Khaleesi? A mummer's dragon, you said. What's a mummer's dragon, pray?"
"A cloth dragon on poles," Dany explained. "Mummers use their in their follies, to give the heroes something to fight." (Dany V, ACoK 63)
So in a nutshell, the mummer's dragon is a puppet. If the mummer wants the puppet to go left, it goes left. And if he wants it to go right, it goes right. The puppet is something a mummer can manipulate to do what it wants it to do. It doesn't make the dragon a fake, and as we find out from Jon Connington, Aegon is not nearly as biddable as Young Griff was. So the puppeteering (pretty sure it's not a word) will be more difficult moving forward, I think.
But let's forget the whole Blackfyre v. Targaryen debate for the purpose of this thread and look at what we find out about Aegon in the HotU. Because we have to take him into consideration for the prophecy because he is part of it.
Viserys, was her first thought the next time she paused, but a second glance told her otherwise. The man had her brother's hair, but was taller, and his eyes were a dark indigo rather than lilac. "Aegon," he said to a woman nursing a newborn babe in a great wooden bed. "What better name for a king?" "Will you make a song for him?" the woman asked. "He has a song," the man replied. "He is the prince that was promised, and his is the song of ice and fire." (Dany IV, ACoK 48)
And we get more from Maester Aemon;
"No one ever looked for a girl," he said. "It was a prince that was promised, not a princess. Rhaegar . . . I thought . . . the smoke from the fire that devoured Summerhall on the day of his birth, the salt from the tears shed for those who died. He shared my belief when he was young, but later he came to believe that it was his own son who fulfilled the prophecy, for a comet had been seen above King's Landing on the night Aegon was conceived, and Rhaegar was certain the bleeding star had to be a comet." (Sam IV, AFfC 35)
If Rhaegar shared the prophecy with Jon Connington and what he believed the same way he did with Maester Aemon, then Connington might put Aegon forward as the prince that was promised because of Rhaegar's own belief. And this is what matters for this triad in the end, I think. Everything seems to indicate that Rhaegar was wrong about that. But if Jon Connington takes Rhaegar's word for it, then the error grows into something else.
If Aegon got his head smashed against the wall, then the promise dies with him. And if he is an impostor, he still doesn't fulfill the requirements of the prophecy. And if he's the real Aegon, he's still a long way from being the fulfillment of the prophecy, which takes us to the final part of the triad;
From a smoking tower, a great stone beast took wing, breathing shadow fire.
The Azor Ahai prophecy as given to us by Melisandre;
When the red star bleeds and the darkness gathers, Azor Ahai shall be born again amidst smoke and salt to wake dragons out of stone. (Jon X, ADwD 49)
And Maester Aemon tells us this;
Daenerys is the one, born amidst salt and smoke. The dragons prove it." (Sam IV, AFfC 35)
So waking dragons from stone is on the checklist for fulfilling the prophecy and this part has not been delivered yet by Melisandre. She gave Stannis the salt, the smoke, when the red star was bleeding, but she gave him a glamored sword instead of dragons.
Dany has woken dragons from stone, thereby literally fulfilling the prophecy. Meanwhile, Aegon as Tyrion observed has no dragons, and neither does Stannis for that matter.
For a long time, I wondered what this part of the vision mean, great stone beast taking wing, breathing shadow fire. But then, when I came at it from the Azor Ahai prophecy, it actually bookends the first part of the vision about the blue-eyed king nicely, because it is part of the prophecy, the part about waking a stone dragon.
I went back in the text and checked for stone dragon references.
The very first time it's brought up in the text is in Dany IV, AGoT 36. Dany refers to her eggs (the stone dragons within) and how she draws strength from them. After that, we get references to stone dragons when we get descriptions of Dragonstone. Otherwise, all the talk of stone dragon comes from the people around Mel and Mel herself. We find out she wants to give Edric Storm over to the flames because of his king's blood to wake the stone dragon.
"This talk of stone dragon . . . madness, I tell you, sheer madness." (Alester Florent, Davos III, ASoS 25)
"Give me the boy, Your Grace. It is the surer way. The better way. Give me the boy and I shall wake the stone dragon." (Davos IV, ASoS 36)
"Your brother's blood," Melisandre said. "A king's blood. Only a king's blood can wake the stone dragon." (Davos IV, ASoS 36)
Ser Axell went to one knee. "On bended knee I beg you, sire. Wake the stone dragon and let the traitors tremble." (Davos V, ASoS 54)
And it goes on.
In her own chapter, Mel has a vision of dragons;
Through curtains of fire great winged shadows wheeled against a hard blue sky. (Melisandre I, ADwD 31)
So she does see actual dragons in her fires. We don't know if she thinks that this means she will be successful in waking the stone dragon. We don't have her opinion on the matter.
I think the beast in the vision from the HotU will be Melisandre's own creation. A stone dragon (and maybe not quite so literally as that), breathing shadow flame. And if this stone beast is breathing shadow flame, then the odds are, this will not be a real dragon, in the same way Lightbringer is not the real Lightbringer. Melisandre is good at faking.
I think the last part of the triad is about how far Melisandre will go to make the prophecy happen.
"No," the old man said. "It must be you. Tell them. The prophecy . . . my brother's dream . . . Lady Melisandre has misread the signs. Stannis . . . Stannis has some dragon blood in him, yes. His brothers did as well. Rhaelle, Egg's little girl, she was how they came by it . . . their father's mother . . . she used to call me Uncle Maester when she was a little girl. I remembered that, so I allowed myself to hope . . . perhaps I wanted to . . . we all deceive ourselves, when we want to believe. Melisandre most of all, I think. The sword is wrong, she has to know that . . . light without heat . . . an empty glamor . . . the sword is wrong, and the false light can only lead us deeper into the darkness, Sam." (Sam IV, AFfC 35)
So the sword is wrong, the dragon breathing shadow flame is wrong, Stannis is not Azor Ahai and neither is Aegon.
Sam hesitated a moment, then told his tale again as Marwyn, Alleras and the other novice listened. "Maester Aemon believed that Daenerys Targaryen was the fulfillment of a prophecy . . . her, not Stannis, nor Prince Rhaegar, not the princeling whose head was dashed against the wall." (Sam V, AFfC 45)
Slayer of lies
And it just occurred to me that Sam's nickname is "Slayer," and this is exactly what he doing here, slaying the Stannis lie and the Aegon lie.