I've been rewatching The Dragon Prince from the beginning, and one thing I've really been paying attention to is Viren's choices and mistakes and how they lead to his eventual downfall.
A big one is the "secret missions" he gives to his children when he sends them out to find the princes: Soren is told to "take care of" the princes, and Claudia is tasked with recovering the egg at all costs.
On the surface level, it makes sense. The jobs given fit the skillsets of the children he gives them to: to the warrior, he gives the role that might require physical violence, while he sends the mage to get the magical object.
But it fails to take into account who Soren and Claudia really are.
Between the two siblings, Soren is the one with the moral compass. It doesn't always point true north, but it's pretty solid where it counts. Killing innocent children-- killing his friends-- is something he can't do. His conscience won't let him.
Claudia, on the other hand, prioritizes her family above everything else. Choosing the egg over Soren-- picking magic over blood-- isn't a choice she could ever make and live with herself afterwards.
These missions were always doomed to fail.
If Viren had switched the assignments, they would likely have suceeded. If he'd told Claudia that the princes actually coming home would put their family's future at risk, the boys never would have survived. If he'd warned Soren of a powerful weapon that couldn't be allowed to fall into enemy hands, the soldier in Soren would have done whatever he could to bring it back, no matter what he had to give up along the way.
Viren's mistake was that he fundamentally misunderstood his children's true natures.
He knew what they were, but not who they were. And that's why he failed.