Is the inscription on this known dagger completely new mythology for the TV series House of the Dragon?

Question

In the Game of Thrones spin-off TV series, House of the Dragon it has been revealed in the first four episodes that King Viserys Targaryen has knowledge, both through dreams and by an inscribed family heirloom in the form of a dagger, that the Targeryen line will produce "The Prince That Was Promised" and he'll take on the realm's biggest threat (The White Walkers) sometime in the future. He is also passing this knowledge onto his daughter Rhaenyra, believing it is their family's responsibility to know this burden.

1.04 (King of the Narrow Sea) Transcript:

King Viserys: That dagger once belonged to Aegon the Conqueror. It was Aenar's before that. And before that... well, it is difficult to know. Before Aegon's death, the last of the Valyrian pyromancers hid his song in the steel.

Rhaenyra: "From my blood... come the Prince That Was Promised... and his will be the Song of Ice and Fire."

King Viserys: The responsibility I have handed to you, the burden of this knowledge... it is larger than the throne, the king. It is larger than you and your desires.

Viewers of Game of Thrones already know this most likely to be Jon Snow, and the events of Game of Thrones known as The Long Night. Viewers of Game of Thrones also likely recognize King Viserys' dagger as "Catspaw", first seen with Lord Baelish, then Lord Varys, and eventually becoming an important weapon of Arya Stark... . Now while I have a lot of questions about why this would be relevant to the events of the 'Dance of the Dragons' and/or may presume there could be some set-up with the upcoming Jon Snow TV series that is currently in development, my actual question is more about the additional mythology of Catspaw being a Targaryen family heirloom that is inscribed with this prophecy.

Is this additional mythology of "Catspaw" created specifically for the TV series, or was this mentioned in previous source material that I missed?

Answer 1

It does not appear in any source material, but is based on the information given to the showrunners by George R.R. Martin:

At a press junket with Insider, Popsugar, Metacritic, and Gizmodo, Ryan Condal stated that the information about Aegon having a prophetic dream of the White Walkers' return came from George R.R. Martin himself. He told Condal during the early writing process, at the meetings where they were figuring out the "architecture" of the series as a whole (...):

"That was the detail that George actually gave us early in the story break — the idea that Aegon the Conqueror was himself a dreamer and that's what motivated the Conquest - Which he mentioned casually in conversation, as he often does with huge pieces of information like that. - He saw a vision of the White Walkers coming across the wall and sweeping over the land with cold and darkness. And but it never made the history books because [Aegon] never told anyone, or at least the people they told didn’t tell the history to the history writers. So it’s in George’s head."

Martin later confirmed that in an interview:

George R.R. Martin himself made a guarded statement about the prophecy in a co-interview with Ryan Condal in Vanity Fair (...):

"I don't want to give too much away, because some of this is going to be in the later books, but this is 200 years before the events of Game of Thrones. There was no sell-by date on that prophecy. That's the issue. The Targaryens that know about it are all thinking, Okay, this is going to happen in my lifetime, I have to be prepared! Or, It's going to happen in my son's lifetime. Nobody said it's going to happen 200 years from now."

It should be noted that, while Aegon's dream and the prophecy was the author's idea, the inscription on this particular dagger seems to be an invention of the showrunners, probably just to add another Game Of Thrones reference.