Personally It sounds just fine for me, If I were reading your novel the emotional impact would be quite the blow and therefore if I liked your characters I would want to keep reading to find out what happens next.
If on the other hand you want to soften the impact add foreshadowing but depending on your audience you should vary your level of foreshadowing for instance extremely genre savvy people will find the smallest bit of foreshadowing and act accordingly (even if you were not upfront about the novel being a trilogy) larger foreshadowing however means that you are going to pull something even more unexpected (depending on how you do it, ps don't insult peoples intelligence).
Also remember this is your novel, If you have to do something that is unoriginal do so your way. Do not be original just to be original as it tends to alienate people no matter if you do it right or wrong.
Hypothetical, for example, other novels you have read have had dragons that was boring so you made your dragons centaur-ish. Think to yourself would this make your creatures weird and uninteresting or weird and interesting to your audience? because chances are people are still going to think it is weird based on their culture (literary culture especially) and experiences.