The Moon and the Sun

They meet each other without warning, within the secrets, affairs, schemes and dark passages of the castle, and collide into fireworks. They will melt and crash together, evolving with and against the other, and will desperately try to resist the inevitable sinking flow of life

OR

the one where Clarke is a princess with heavy reponsabilities towards the kingdom while competing with her twin brother for the future throne, and where Lexa gets chosen to be part of the Elite of Arkadia's kingdom.

ALSO

I promise you the story is better than the summary