The title explains it pretty well... Well it's about a girl who is apprenticed to Will and it's right after the Emperor of Nihonja (did I spell that right) All rights go to the Awesome John Flanagan

I had to remind them who I was. Send a clear message to all who pursued me that I was Grimalkin, the witch assassin of the Malkin clan. I had run for so long that they had grown disrespectful. I had to teach them fear again... 'I am Grimalkin and I could kill you all!' Grimalkin has made it her lifetime ambition to destroy the fiend, avenging the brutal murder of her son. Having grudgingly joined forces with the Spook and his apprentice, Tom, and assisted them in binding the fiend, she is now on the run. Her mission is deadly! In order to destroy the fiend's spirit for ever, Grimalkin must keep his severed head in her possession. No matter who-or what-comes after it... This short story is a modified version of Joseph Delaney's SPOOK'S I AM GRIMALKIN.