Julia realized what his plan had been all along. She fell into their trap when he wrote a letter to her and the moment that Runera threatened her. It was never about making her betray Arthur but using her to make Arthur lose control.
It was ironic that Julia warned him about losing control before, only to end up being the reason behind it. She trusted Yuvan with everything, and he turned out to be her enemy all along.
However, it was too late for regrets now. As she witnessed what had replaced Arthur, she knew these Twelve Masters got what they wanted. Arthur has seen what happened to her, which triggered him to let Eragon out.
Julia could feel the ground tilting beneath her, but she could not move. Beside her was the cold corpse of her father, and in front of her was the other man she cared about, losing control. Julia felt helpless.