HotD Fans, Here's Your Proof Viserys Was Right All Along (As If We Needed One)
Dragons will be the downfall of House Targaryen.
When asked about the Targaryen connection to dragons in House of the Dragon, Viserys described the idea as 'an illusion' and called it 'a power that men should never have trifled with'. He even went on to say that this belief could be the downfall of the Targaryen dynasty.
And while there are many instances of dragons being faithful to the commands of their riders, there are also times when the beasts have gone against what they have been told to do.
You only have to look at the way Vhagar killed Arrax and Lucerys to see that the king's words were true. In that particular battle, both dragons ignored their riders - who were, of course, young and inexperienced. But the death of Lucerys is an event that brings the Targaryen civil war ever closer.
It's also worth noting that it's believed that dragons can sense the feelings of their riders and mimic those emotions. Applying this theory to the aforementioned scene, we see that Lucerys is panicked by Aemond's pursuit, and this may be what causes Arrax to attack Vhagar.
Similarly, while Aemond has no intention of killing Lucerys, he does want revenge. This may be what makes Vhagar so aggressive.
Earlier in the series, we saw Vhagar reluctantly kill her former rider, Laena Valaryon, who had ordered her own death after complications in childbirth. And it was only after Laena managed to compose herself and look her dragon in the eye that her order was obeyed.
This suggests that an immense amount of mental strength is required to control dragons. Consequently, riding them in combat situations is a risky move, regardless of the strength of the bond between dragon and rider.
One thing that has shone through all parts of the divided family in the show is the fact that there is a lot of hatred and very little emotional restraint. And it seems Viserys was right. Controlling a dragon when you're taking it for a ride or simply using it to cover long distances may indeed be something some Targaryens can do.
Maintaining control when things get unruly is another matter entirely. That will be part of the 'doom' Viserys feared. But then we already knew that.