This book was a fantasy book based in a medieval time period with magic. The start of it was the main character as an old man who was a retired spy/assassin for a king reminiscing on his past adventures and accomplishments. He was a stable boy or servant and somehow came into possession of a magic stone/crystal that granted him a power(I believe this one was invisibility). I think there were a finite amount of these stones and each had different powers and I think he needed to make an agreement with the supernatural beings that the stones draw their powers from. The stones drew their power from creatures from another realm/world and I think the cost of using them was pretty high, possibly his soul or something along those lines. One of his first exploits was as a page to a knight and he was in the king's army assaulting a castle, he used the stone to turn invisible and infiltrate the gatehouse to unlock the gate to the castle and let the army in. It was probably about 10 years ago when I read it, but I believe it was a young adult fantasy book or just a normal one.