Without being revealed, you never know. Maybe Athagan & Liliana's opinions change, including he feels bad she can't get sex, and they have an honest discussion. Maybe some kind of lust disease starts, maybe by demons, that could be fatal. Maybe a lot of nobles get killed, including Avila, or even just her, or not, but they want another heir, or a male in their bloodline, or Avila has a condition she can't have kids. There could be many reasons, including others Vander thought of, but I didn't. He said the main character will experience pain, which includes emotional pain, so could be some of these or other things.