This is a brilliant move for Chapter 8. Shifting the perspective to the "villains" (Kaulie and Gavin) adds so much tension because the reader realizes that both sides are now hunting Sindel and Dominic.
Your addition of the Harpy’s true motive is perfect—it turns her into a much more dangerous "third player" who is manipulating everyone for her own twisted jealousy.
Here is the polished version of Chapter 8, tightening the dialogue and making the contrast between the two kings even sharper.
CHAPTER 8: THE KINGS OF SHADOW AND LIGHT
In the marble halls of Clover Valley, King Kaulie paced with a frantic, heavy energy. Aiden stood before him, his head bowed in shame after delivering the news: the Prize had been taken by the Vampire Prince.
"How could she escape?" Kaulie spat, his voice echoing off the high ceilings. "She was shackled! She was in the strongest cell!"
"I apologize, Your Majesty," Aiden said quickly. "I locked the door myself. But the vampire... he didn't use a key. He tore the iron bars from the very stone. The Slayers followed the tracks as far as the border of Darkness Falls. He took her straight to the King of Shadows."
"This is a catastrophe!" Kaulie shouted, slamming his fist into a stone pillar. "Gavin will kill her! She is the Prize—the only one who can end this—and she cannot defeat an entire legion of vampires alone!"
Aiden stood silent, weathering the King’s verbal blast. Kaulie returned to his pacing, his mind churning. He knew only fragments of the prophecy; he believed the Prize would slay Gavin’s son, but ultimately lose, allowing the Phoenix to be reborn. He didn't realize that the "forbidden love" unfolding in the woods was the true path to fulfillment. He feared his kingdom was on the verge of ruin, and all his years of protection would be in vain.
"Change the plan," Kaulie growled, his eyes narrowing. "We won't wait for her to fulfill a prophecy I no longer trust. We ambush them. I will lead the Slayers into Darkness Falls myself. I will end my brother, take his crown, and rewrite the fate of this world with my own hands."
Meanwhile, miles away in the jagged spires of Darkness Falls, Gavin was receiving a similar report, though the air here tasted of blood and terror.
"What do you mean he’s gone?" Gavin roared at the messenger.
"Sire, Prince Dominic never reported back," the soldier stammered, trembling. "He was ordered to bring the King’s head or the Prize. We searched the border until the sun forced us back. There is no sign of him. He must have been captured or killed by the Slayers."
Gavin’s eyes turned the color of fresh arterial blood. In a fit of rage, he struck the messenger with a backhand so powerful it sent the vampire spiraling across the room. He hit a stone pillar with such force that the ancient granite crumbled.
"Dominic, you fool!" Gavin growled into the night. "I thought you were smart enough to slip their nets. If my own seed cannot bring me the Prize, then I will find her and take her head myself!"
Just as the King reached the peak of his tantrum, a familiar, screeching cry cut through the air. He stiffened. He hadn't heard that sound in decades. He looked up to see a massive shadow blotting out the moon.
The Harpy descended, her talons clicking sharply against the stone floor of the balcony. As she landed, her feathers melted away, morphing into the tattered rags of the haggard, rot-smelling crone Gavin remembered from the Cave of Echoes.
"You," Gavin said. He was no longer the terrified nineteen-year-old boy who had traded his soul for a crown. He was a monster in his own right.
"You look well, boy," the witch cackled. Her face was a map of wrinkles, her mouth pulled back in a jagged, single-toothed grin. She hobbled toward him, her body twisted and humped. "I am proud of you, Gavin."
"We meet again, Harpy," Gavin replied, shaking his head as if to wake from a nightmare.
The witch circled him like a vulture. "I know where your favorite son is at this very moment," she whispered, her voice like dry leaves.
"Where? Where is Dominic?" Gavin asked, a flicker of hope rising—not for his son’s life, but for the success of the mission.
"He has joined with your sworn enemy," the Harpy hissed, twisting the truth into a lethal lie. "He has become a traitor. He doesn't want the Prize for you—he wants her for himself. He plans to use her power to take your crown and rule as the new King of Vampires."
Gavin’s jaw clenched so hard the bone nearly snapped. "He would dare?"
"He has already begun," she lied.
"That settles it," Gavin roared, his cape billowing like a shadow. "I will end the Prize, and I will take my son’s head along with hers!"
The Harpy’s milky eyes glowed with hidden triumph. She didn't care about Gavin's crown or Kaulie's kingdom. Her motive was much older. By manipulating the brothers into killing Sindel and Dominic, she would ensure her sister Aurora remained a song less, imprisoned firebird forever. Once the light of the Phoenix was extinguished, the Harpy would bring an eternal darkness to the earth. She would be the only female left in a world of men, a dark queen to be worshipped and lusted after, ruling a world of shadow where love was finally dead.
I love how this sets up a "race" to see who finds them first! For Chapter 9, should we go back to the cave where they are waking up, or do you want to keep following the Kings as they prepare their armies?