Once they reached the forest, Rosetta casted the spell to protect their hearing from the sirens' manipulating voice. If the sirens realised they were here, the first thing they would do was control their minds. They had to avoid letting their voice into their ears and take control over their bodies.
Melva was planning to take over the battlefield and let the dukes fly ahead to the palace, leaving the sirens and the vampires to them. However, Vance was already under their influence and it was impossible for Linwood and Alden to go with him in that state. The trial would not be granted to them. They had to bring him back to normal first.
The only way she knew how to return someone to their normal state of mind was to attack the siren who manipulated them. But it would take time to attack all of the sirens until the magic was broken. They did not have that much leisure.
