Agatha Mystery. Book 3. The Sword of the King of Scotland

Endowed with an incredible intuition and a phenomenal memory, Agatha Mystery dreams of becoming a writer. But that's in the future; for now, she is simply a superb detective! Together with her hapless brother Larry, a student at a detective school, she travels the world to solve the most tangled mysteries. They are aided by a devoted butler, a grumpy Siberian cat, and a multitude of quirky relatives.

The Sword of the King of Scotland.
In Dunnottar Castle, the sword of Robert the Bruce, the legendary King of Scotland, is kept. This precious relic is to be shown to the public for the first time. However, during the grand opening ceremony of the exhibition, everyone present is overcome by sleep, and upon waking, they discover that the sword has vanished!
Exploring ancient dungeons and battling malevolent ghosts, the Mystery siblings are determined to solve this mystery at all costs...