Warning: SPOILERS for Game of Thrones season 7, episode 5 ahead!
Dragons have come again to Westeros and they're making quite the impression. Last week's episode of Game of Thrones demonstrated the incredible power they wield in battle, and in this week's episode, Daenerys used Drogon for a grisly execution that sent every surviving enemy soldier to their knees, swearing fealty to the Dragon Queen.
But these dragons inspire more than fear -- they are magic come to life and just about all who witness them (and live) become awestruck in their presence. This makes Daenerys' dragons so much more than weapons, but symbols of her power and right to rule the Seven Kingdoms, just as her ancestors -- themselves dragon riders -- did for generations.
However, Daenerys is but one person and she has three dragons. They may all be her children, but the very fact that she only brought along Drogon to attack the Lannsiter army seemingly implies she can comfortably control only one dragon. She and Drogon clearly also share the strongest bond as he is the only one she's actually ridden. So while that's all well and good, it does leave two more dragons which can potentially be tamed and ridden. The big question here is, of course, who?
The idea that Daenerys' three dragons will eventually have three riders comes from a vision Daenerys experiences while in the House of the Undying in Qarth. This would the sequence in the television series where Daenerys has visions that she is walking through a burnt out throne room in King's Landing and later sees her dead husband, Khal Drogo, and their stillborn son, Rhaego. In the novels this section unfolds differently and Daenerys walks through several rooms of visions, some obviously referencing important moments -- like the Red Wedding -- while others are more cryptic and strange -- like a naked woman being ravaged by four dwarves.
One of these scenes, however, is believed to show her older brother, Rhaegar, speaking to his wife about the future of their son, Aegon. He refers to him as "the prince that was promised" and says that his will be "a song of ice and fire." Additionally, Rhaegar says that "the dragon has three heads" while seeming to stare directly at the observing Daenerys. Now, prophecy is largely about how it is interpreted, and already Game of Thrones has made it known that prophecies can be wrong just as often as they are right. While it isn't exactly clear from this vision just how the dragon having three heads ties in with the "prince that was promised" prophecy (also believed to be related to, if not the same as the Azor Ahai prophecy), but it reinforces an image which is strongly connected with Targaryens and their dragons.
The Targaryen sigil is a three-headed dragon and it was Aegon and his two sisters, Rhaenys and Visenya, who conquered Westeros and forged the Seven Kingdoms from the backs of their three dragons -- Balerion, Meraxes and Vhagar. Rhaegar is therefore drawing a connection between his ancestor and the legendary warrior, Azor Ahai, believing that from his line will come the prince that was promised. Judging by the scene Daenerys witnesses, Rhaegar believed the three heads would be his children, two of whom he named Aegon and Rhaenys. "There must be one more," Rhaegar says, at the time not knowing that his wife, Elia Martell, would only be able to bear him two children. Of course, by now every viewer knows Rhaegar did eventually have a third child and it is with this reveal that the possible prophecy begins to gain traction.