Complex sentence: Sofia is horrified to learn that they are going to meet Shuriki since Roland sent a message ahead telling her they were coming since it is the custom to greet the King or Queen when visiting a new Kingdom.
Split into simpler sentences: Sofia is horrified to learn that they are going to meet Shuriki since Roland sent a message ahead telling her they were coming. It is the custom to greet the King or Queen when visiting a new Kingdom.