I had a feeling I knew what was going to happen. I knew, for one reason or another, it would be Aranwen's last song. Either she failed, in which case she likely wouldn't live to sing again, or she'd be successful, and throw away her sword for the pain and suffering she caused to people following it.
The oath that broke with her heart's blade was the oath that she'd care for Kithran as her own child, however she could even if she was never to admit it for Kithran's comfort. To fill the place of the woman who abandoned her. Even through the haze of stun, she could feel when Kithran stabbed Ch'dau. In that instant, in her heart, she knew there was no way to salvage that oath. She failed to uphold the same oath that Saeriel had made, an oath to protect.
If she should be able to move once more, she will do the only thing she thinks she might yet have a chance to - to strike Davena down, regardless of her own safety. No more song; she lost it, all she has left is the cold steel in her hands.