Complex sentence: There was a party at a royal castle to celebrate the prince's birthday, which was interrupted by the three-headed mouse queen and her son, who both behaved very rudely and refused to leave or improve their manners.
Split into simpler sentences: There was a party at a royal castle to celebrate a newborn prince. It was interrupted by the three-headed mouse queen and her son, who both behaved very rudely and refused to leave or improve their manners.