ASOIAF: The Rise of House Ashenblade.

When an old assassin woke up in the body of the young heir to House Ashenblade, he never expected to face a raid the moment he opened his eyes. But, well... blood had never been unfamiliar to him. Everything he had learned in his past life would now serve a single purpose. One simple to explain, and far more difficult to achieve: to rise through the ruthless game of power in Westeros. But first, he had to survive. His lands were sparsely populated. His house was poor. His enemies were stronger than him. And he had barely two silver coins to his name. A system? Well. That solved a lot of problems. — Description from novelcodex.org