This is something of a game but also an exercise in creating game content. The idea is to create a character, either one from a fairly well-known source outside of the 'Dresdenverse', or new one that doesn't appear in the books, but has a High Concept and description that fits. Include names, aspects, skills, powers, etc. - everything that would be needed for someone to play the character or for a GM to 'borrow' and use as an NPC. If you have an idea for a new Character Design Challenge, copy this introductory text and paste it into a new thread.
This time, let's try Stormbringer, the demonic black blade wielded by Elric, from Michael Moorcock's series. The sword itself is a character, similar to the way a Denarian's coin is. It could be handled with the Demonic Co-Pilot power, but what about looking at things from the sword's perspective? If you can drink in the lives of those you kill and dole that power out until your wielder becomes dependent upon it, they'll eventually become dominated compel-bait. If one wielder dies, no matter, any fool capable of lifting the blade can make a suitable replacement, in time... the sword itself is the villain.
Happy Hacking!
Thanatos