How did Murtaugh get so much power so fast?

I would like to ask the question: How did Murtaugh gain so much power so fast? I have a very good answer to this question, but what do you think?

EDIT: Okay, here it is. Read back to the fight between Murtaugh and Eragon in the second book. Remember how Eragon couldn't identify him because there seemed to be more than one person in him? No read even further back, to when Galbatorix kills the leader of the riders. It says that power flowed through his veins from the kill. Both Galbatorix and Murtaugh have been stealing souls and using their power. This is why Galbatorix's strength grows every year, and why Eragon must go to the Vault of Souls, or what I believe to be the Rock of Kuthian. This may be a bit of a stretch, but I think that Riders stockpiled their souls in this vault when they were killed and Galbatorix rose to power. Continue to build on this if you like.