Fiction Assassin story/ Orbs of power

I read a story in high school about 4-5 years ago that was about the sorta medieval times when a boy was taken in by a master assassin because the boy wanted to be one. Some of the vague memories of it is that its part of a series where there were orbs of power; where in the first one the master assassin had the silver orb which granted them certain powers (I.E invisibility, super speed, ect). I also remember the master assassin taught the boy and had him preform deeds, one of which I think was assassinating the king or his daughter?. At the end of the story the boy has to fight his master and kills him, even though he didn't want to and takes the orb then sets out to find the others at his masters dying request.

It was paperback, and I think the cover had a purple robed assassin on it? with a white background and black lettering.

UPDATE: I found it after some deep, and I mean DEEP digging... The book/ series I was looking for was the Night Angel Trilogy the book was the way of shadows.
http://www.librarything.com/work/5381591/book/128691877

Glad you found it! Thanks for coming back and letting us know.

Touchstone add: way of shadows