Old friends.

Cassandra is the second oldest and the Princess of Powys. The kingdom right next to Camelot. She used to be best friends with Arthur Pendragon until he did something that made her hate him and hold a grudge. She's a witch and her and her brother and sisters are forced to take refugee in Camelot after a rebellion in their kingdom. The only problem is that magic isn't allowed in the Camelot so Cassandra has to keep her secret under wraps while trying to fight magic from destroying her new home with some help from another warlock.