The Dancing Rapier was a magical sword that belonged to Sinu the Red, before being taken from his Crypt by Rowan Grey, sometime in the year 1707. It possessed the magical property (or was it a physical atribute?) to fly around in the air and attack whomever it's user commanded1.
The Rapier was found in Sinu's Crpyt, where it, along with a myriad of other magical items, was pilfered and examined by the Ätlän-tã Fælcons. After determining what it was, and what it could do, Rowan decided to take it for herself1. It remained with her throughout their adventures, and she kept it during the Seven Year Gap, during which she founded The Tower of Grey with Toby Treacletart2. She would use it frequently while she and her team searched for the missing Lannie, and Prince Thom Harper.
It was lost to her when she and the others were imprisoned in Grull, but she found it again when they ransacked the Impound room.
Just the episodes used. She has it in every episode after the first one.