Unknown, likely Firewyrms from Old Valyria
Secrecy
Unfortunately for us and posterity, Septon Barth was sworn to secrecy by Grand Maester Bennifer, so not much is left to account for exactly what happened.
Barth did though succumb and ended up writing in his diary (which was found many years later) in secret.
“I do not know. I do not want to know. If this makes me a faithless
septon, so be it. Grand Maester Benifer and I have agreed to tell no one
all of what we saw and experienced in his chambers as that poor child
lay dying…not the king, nor the queen, nor her mother, nor even the
archmaesters of the Citadel…but the memories will not leave me, so I
shall set them down here. Mayhaps by the time they are found and
read, men will have gained a better understanding of such evils.
Barth's Theories
It should be noted that although Barth made some initial theories, this unfortunate event took a toll on him and he became almost obsessed with discovering the secrets of Old Valyria and the origins of Dragonkind and Wyrmkind, leading him to eventually write Dragons, Wyrms, and Wyverns: Their Unnatural History. (more of its relevance will be revealed...)
Initial Theories
In those initial days after Aerea's death, Barth writes of the horrors in Aerea's body:
there were things inside her, living things, moving and twisting,
mayhaps searching for a way out, and giving her such pain that even
the milk of the poppy gave her no surcease.
...
The simplest way to say it is that the poor child was cooking from within. Her flesh grew darker and darker and then began to crack, until her skin resembled
nothing so much, Seven save me, as pork cracklings. Thin tendrils of smoke issued from her mouth, her nose, even, most obscenely, from her nether lips. By then she had ceased to speak, though the things within her continued to move. Her very eyes cooked within her skull and finally burst, like two eggs left in a pot of boiling water for too long.
-Septon Barth, Fire & Blood
Image in spoiler tags due to its gruesome nature:
-Ibid.
...
The things... Mother have mercy, I do not know how to speak of them... they were... worms with faces... snakes with hands... twisting, slimy, unspeakable things that seemed to writhe and pulse and squirm as they came bursting from her flesh. Some were no bigger than my little finger, but one at least was as long as my arm... oh, Warrior protect me, the sounds they made...
-Ibid.
Barth becomes convinced that after Aerea "claimed" Balerion, rather than her guide him to King's Landing, he took her to Valyria:
“The princess might well have intended to fly to King’s Landing, just
as her mother suspected.
...
It is one thing to leap upon a dragon and quite another to bend him to your will, particularly a beast as old and fierce as the Black Dread. From the very start we have asked, Where did Aerea take Balerion? We should have been asking, Where did Balerion take Aerea?
...
“Only one answer makes sense. ... he is mayhaps the only living creature in the world that knew Valyria before the Doom.
...
“And that is where he took the poor doomed child clinging to his back.
-Ibid.
Ok, so Balerion most likely took Aerea to Valyria, because he missed home or something.
Not much is known of Valyria since the Doom. What everyone agrees with is that it's nothing good.
Barth goes on to speculate:
“What befell her on Valyria I cannot surmise. Judging from the
condition in which she returned to us, I do not even care to
contemplate it. The Valyrians were more than dragonlords. They
practiced blood magic and other dark arts as well, delving deep into
the earth for secrets best left buried and twisting the flesh of beasts and
men to fashion monstrous and unnatural chimeras. For these sins the
gods in their wroth struck them down. Valyria is accursed, all men
agree, and even the boldest sailor steers well clear of its smoking
bones… but we would be mistaken to believe that nothing lives there
now. The things we found inside Aerea Targaryen live there now, I
would submit…along with such other horrors as we cannot even begin
to imagine.
-Ibid.
So he seems pretty sure that:
- Balerion took Aerea to Valyria
- Valyria is full of horrors (as it was even before the Doom)
- The things in Aerea are in Valyria
Later theories
Barth becomes a little bit obsessed with finding the origin of Dragonkind and also the secrets of Old Valyria, to his detriment. Many of his discoveries and theories led to him being mistrusted and likened more to a sorcerer than a Septon.
Some of his related theories include:
In Septon Barth's Dragons, Wyrms, and Wyverns, he speculated that the bloodmages of Valyria used wyvern stock to create dragons. Though the bloodmages were alleged to have experimented mightily with their unnatural arts...
-The World of Ice and Fire - Beyond the Free Cities: Sothoryos.
In such fragments of Barth's Unnatural History as remain, the septon appears to have considered various legends examining the origins of dragons and how they came to be controlled by the Valyrians. The Valyrians themselves claimed that dragons sprang forth as the children of the Fourteen Flames
-The World of Ice and Fire - Ancient History: The Rise of Valyria
The "Fourteen Flames" is the name given to the fires of mt doom Valyria's volcanic mountains and the mines dug deep into them by the Valyrians.
Septon Möoz'* theory - Firewyrms
Here, I will attempt to bring it all together into a solid theory that it was in fact the Firewyrms that are mentioned to Arya at the House of Black and White.
The harsh and oppressive nature of Valyria's deeps.
As previously pointed out, Septon Barth mentions the cruel and tough nature of Valyria and its deep places, he also speculates as to the origin of the dragons having sprung from there, and also their connection with the Firewyrms.
We are also introduced to the story of the Faceless Men and their guild, originating from those very depths of Valyria. Furthermore, re-iterating the harsh and cruel environments therein: (please note the similarities to both the conditions within the deeps, as well as the descriptions of the Firewyrms)
"The tale of our beginnings. If you would be one of us, you had best know who we are and how we came to be. Men may whisper of the Faceless Men of Braavos, but we are older than the Secret City. Before the Titan rose, before the Unmasking of Uthero, before the Founding, we were. We have flowered in Braavos amongst these northern fogs, but we first took root in Valyria, amongst the wretched slaves who toiled in the deep mines beneath the Fourteen Flames that lit the Freehold's nights of old. Most mines are dank and chilly places, cut from cold dead stone, but the Fourteen Flames were living mountains with veins of molten rock and hearts of fire. So the mines of old Valyria were always hot, and they grew hotter as the shafts were driven deeper, ever deeper. The slaves toiled in an oven. The rocks around them were too hot to touch. The air stank of brimstone and would sear their lungs as they breathed it. The soles of their feet would burn and blister, even through the thickest sandals. Sometimes, when they broke through a wall in search of gold, they would find steam instead, or boiling water, or molten rock. Certain shafts were cut so low that the slaves could not stand upright, but had to crawl or bend. And there were wyrms in that red darkness too."
"Earthworms?" she asked, frowning.
"Firewyrms. Some say they are akin to dragons, for wyrms breathe fire too. Instead of soaring through the sky, they bore through stone and soil. If the old tales can be believed, there were wyrms amongst the Fourteen Flames even before the dragons came. The young ones are no larger than that skinny arm of yours, but they can grow to monstrous size and have no love for men."
- A Song of Ice and Fire, Book Four: A Feast for Crows. Arya II.
This is a conversation between Arya Stark and "The Kindly Man" in the House of Black and White.
Note the similarity of the description of the Firewyrms and how they behave, in terms of them burrowing through rock and stone, which is basically what they were doing to Aerea. Also, the Firewyrms were reported to be able to breathe fire (which means that they needed a face/mouth).
This is what leads me to believe that the "things" inside Aerea were in fact Firewyrms (or similar creatures) originating from Valyria.
* I am not actually a Septon
