I kinda like your *long live* idea but I want to put a slight change. Of cause my idea turns the whole idea a lot darker.

When the inviders kill the king and queen of the kingdom and see character A, out of spite and cruelity, they take the prince/ princess prisoner and treat him/her much like a slave.

I kinda like dark twists lol Also I'd like to do character A as in play the role of a young prince.