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?