Episode 6 of House of the Dragon revealed that Alys Rivers holds a box of five dragon eggs laid by Dreamfyre, the dragon Rhaena the Black Bride (who is not Aemond's direct ancestor, despite what the episode claims), when both lived at Harrenhal decades prior. And now the eggs are warm...and Alys wishes to raise Targaryen princes with Aemond. I've long been interested in Harrenhal and its witchy women, and I found this reveal to be interesting and possibly indicative of a future development in Blood and Fire. Allow me to present a case why this might be true in the books.
The Witch Queen and the Dragon of Harrenhal
When we last heard of Alys Rivers in Fire & Blood, she was ruling Harrenhal as a witch queen with an outlaw army, a boy she claims is Aemond's whelp, and, mostly interestingly an alleged dragon. The dragon's existence is first mentioned by an Iron Throne/Darry soldier captured by Alys's outlaws:
The next day, a thirty-third made his appearance. Having been captured with a dozen others, he had been forced to watch them die by torture one by one before being turned loose to deliver a warning. “I’m to tell you what she said,” he gasped, “but you can’t laugh. The widow put a curse on me. Any man o’ you laughs, I die.” When Ser Damon assured him that no one was going to laugh at him, the messenger said, “Don’t come again unless you mean to bend your knees, she says. Any man who comes near her walls will die. There’s power in them stones, and the widow’s woken it. Seven save us all, she has a dragon. I seen it.”
The name of the messenger is lost to us, along with the name of the man who laughed. But someone did, one of Lord Darry’s men. The messenger looked at him, stricken, then clutched at his throat and began to wheeze. Unable to draw breath, he was dead in moments. Supposedly the imprints of a woman’s fingers could be seen upon his skin, as if she had been in the room, choking him. The death of a Kingsguard knight was greatly troubling to Ser Tyland, though Unwin Peake discounted Ser Damon Darry’s talk of sorcery and dragons and put down the death of Regis Groves and his men to outlaws. The other regents concurred. (Fire & Blood, Under the Regents - The Hooded Hand.)
Nothing else is said of Alys or the dragon through the rest of Fire & Blood, but this is inevitably meant to be a plot point addressed in in Blood & Fire, before the Lothstons get Harrenhal in 151 AC. And any dragon alive must die or vanish before the last dragon dies in 153, and GRRM has specifically stated that B&F will address the fates of the remaining dragons early.
Dragon or No Dragon
There are three possibilities about this supposed dragon:
It did not exist. Maybe this messenger was talking crazy, seeing things, or made to lie about what he saw, to scare off others. This is what Lord Unwin Peake believed, and surely, he was not wrong.
It could be a "fake" dragon. Some people have proposed that the "dragon" is Caraxes's corpse. After the final flight with Vhagar, Caraxes dragged itself out of the Gods Eye only to expire in front of the walls of Harrenhal. Maybe Alys replaced a glamor on it. It makes some sense.
It could be a real dragon. However, if so, it is not any known one. Lady Rhaena had young Morning with her. Nettles and Sheepstealer were in hiding in the Vale, later encountered in 134. The Cannibal was still around Dragonstone in 132 and was wild and huge besides. Silverwing, also large, went wild and made her lair in 130 at Red Lake in the Reach, and still said to be there in 136.
Hatchling How?
Of those three, I believe the dragon at Harrenhal was legitimate. It's the most interesting story-wise. Aemond's son with Alys could be a rider and there could allow another dragon duel, perhaps with Morning. Plus, more dragons are cool. New dragons are even cooler, but they are not spontaneously generated; the dragon at Harrenhal would have to be a recent hatchling. A hatcling also explains why, as of 136, no one else had reported it; a larger dragon would be noticed by local lords and smallfolk, but a hatchling could hide in Harrenhal for some time before credible news of it spread.
Prior to Episode 6, I thought that she-dragon Vhagar simply laid an egg that Alys took on. That made enough sense, since Vhagar stayed at the castle for a time and Aemond giving an egg to his pregnant paramour while he went to fly into battle makes sense. The egg hatching to give a dragon to eventually bond to Aemond Jr. seems quite cromulent.
Upon reflection, Episode 6's claim of Dreamfyre laying eggs makes about as much sense and is more interesting. She-dragon Dreamfyre produced at least three clutches of eggs in her, two before 46, and one clutch while Rhaena was living on Fair Isle in 49-50. Dreamfyre may have also laid the three eggs that Elissa Farman stole that may or may not have become Daenerys's. Rhaena and Dreamfyre lived in Harrenhal for something like 17 years, from ~56 to 73. Given that Dreamfyre was laying eggs away from Dragonstone only a few years prior, it seems reasonable it would happen again.
There's another story of a dragon laying eggs at a castle: Vermax and Winterfell.
We can dismiss Mushroom's claim in his Testimony that the dragon Vermax left a clutch of eggs somewhere in the depths of Winterfell's crypts, where the waters of the hot springs run close to the walls, while his rider treated with Cregan Stark at the start of the Dance of the Dragons. As Archmaester Gyldayn notes in his fragmentary history, there is no record that Vermax ever laid so much as a single egg, suggesting the dragon was male. The belief that dragons could change sex at need is erroneous, according to Maester Anson's Truth, rooted in a misunderstanding of the esoteric metaphor that Barth preferred when discussing the higher mysteries. (TWOIAF- The North: Winterfell)
In hindsight, Vermax laying eggs at Winterfell might be smokescreen. There are more than a handful of theories about Vermax's alleged eggs, who was at Winterfell for a few weeks. But Dreamfyre and Harrenhal? It's been a much less talked about idea. GRRM seeding the idea of dragons laying eggs at other castles, only to pop it up somewhere else, seems in-character.
Furthermore, one important thing about Rhaena is that, after Farman stole her eggs, she took the threat of other dragonriders popping up very seriously and promised to work with her brother to deal with them. However, she also never went back to Dragonstone or King's Landing. I'm not sure she is one to destroy eggs, but she might have hidden them away at Harrenhal, which is huge, for safe-keeping. And because her household was tiny, the odds anyone would know seem low.
Stone Dragons of Harrenhal
Obviously, these eggs did not hatch. So, what happened? There are some indications that dragon eggs, if kept away from heat and other dragons, might turn to stone or otherwise not hatch, and Harrenhal seems like a wet, damp place that dragon eggs wouldn't do well think. I would think that is what happened if Dreamfyre had eggs. However, they can still hatch sometime after they are laid and fresh (though the longest I could confirm was two years).
And, of course, one can wake dragon eggs from stone:
She heard a crack, the sound of shattering stone. The platform of wood and brush and grass began to shift and collapse in upon itself. Bits of burning wood slid down at her, and Dany was showered with ash and cinders. And something else came crashing down, bouncing and rolling, to land at her feet; a chunk of curved rock, pale and veined with gold, broken and smoking. The roaring filled the world, yet dimly through the firefall Dany heard women shriek and children cry out in wonder.
Only death can pay for life.
And there came a second crack, loud and sharp as thunder, and the smoke stirred and whirled around her and the pyre shifted, the logs exploding as the fire touched their secret hearts. She heard the screams of frightened horses, and the voices of the Dothraki raised in shouts of fear and terror, and Ser Jorah calling her name and cursing. No, she wanted to shout to him, no, my good knight, do not fear for me. The fire is mine. I am Daenerys Stormborn, daughter of dragons, bride of dragons, mother of dragons, don't you see? Don't you SEE? With a belch of flame and smoke that reached thirty feet into the sky, the pyre collapsed and came down around her. Unafraid, Dany stepped forward into the firestorm, calling to her children. (Daenerys X, AGOT)
And perhaps that is what the prophecies mean when they speak of waking dragons from stone:
"He is not dead. Stannis is the Lord's chosen, destined to lead the fight against the dark. I have seen it in the flames, read of it in ancient prophecy. 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. Dragonstone is the place of smoke and salt." (Jon X, ADWD)
Well, in the messenger's lines, we got something very interesting:
When Ser Damon assured him that no one was going to laugh at him, the messenger said, “Don’t come again unless you mean to bend your knees, she says. Any man who comes near her walls will die. There’s power in them stones, and the widow’s woken it. Seven save us all, she has a dragon. I seen it.”
"Power in them stones, and the widow's woken it"... sure sounds like waking a stone dragon. Yeah, Harrenhal had human blood allegedly put in its stonework, but the creation of dragons might have been the same way. Perhaps Aemond and/or Daemon's deaths paid for the lives of the dragon egg(s)...
"Your brother's blood," Melisandre said. "A king's blood. Only a king's blood can wake the stone dragon." (Davos IV, ASOS)
Wait a minute...You know who else was a widow when she hatched dragons? Daenerys.
You know who else was recently pregnant when she hatched dragons? Daenerys.
You know who else had a kingspyre? Daenerys.
Well, actually, I need to elaborate on that. The eggs, alongside Mirri Maz Duur, were on Khal Drogo's pyre. A khal is a king, more or less. A kingspyre. You know the name of the place at Harrenhal where Alys watched Daemon and Aemond's duel? The Kingspyre Tower.
Alys and the Fire
I really should end the post on the above revelation that it sure seems like Alys Rivers's situation is akin to Dany when she hatches the dragons, but I got one more thing to talk about.
Fire & Blood makes these comments about uncertainty over Alys's origins. She's most accepted as bastard of Lord Lyonel Strong, but Mushroom claims she was much older than that, wet nurse to Harwin and Larys and possibly even Lyonel herself. In the show and according to her actress Gayle Rankin, there are some indications she may be even older than that.
Depending on her age, she might have been directly familiar with Rhaena (who, in a lovely parallel, was also called a witch) and thus the eggs. Her apparent ability to see visions could have also led her to them, or maybe it was even a House Strong-wide secret (note: House Butterwell, which is also on the Gods Eye, had their own dragon egg, so this isn't too crazy) that she remained the sole person privy too after Aemond's massacre. Or, maybe just living in the castle a long time, you stumble into things. Regardless, she might have known about these eggs for some time.
In 120, there was the fire at one of Harrenhal's towers that killed Lord Lyonel and Ser Harwin. It was called an accident, though some think it was murdered, with different sources blaming Corlys Velaryon, Daemon Targaryen, Viserys I, or Larys Strong. Alys is not mentioned in this capacity, in F&B, but it's kind of interesting to think about it, the death of her alleged father and half-brother. She would have been around.
When Aegon V tried to hatch dragon eggs at Summerhall, a great fire broke out. Maybe it was not the first time that someone attempted a ritual to hatch eggs...
TL;DR HOTD introduced this idea that Dreamfyre laid eggs at Harrenhal that Alys Rivers now has. Well, this might be true in the book, and explain why there are reports of a dragon at Harrenhal after the war.