http://www.fanfiction.net/s/8661690/1/Merciless
This is one of my favorite fics out there. Not only does it capture the dark theme, it also reminds the reader that just because Corvo spared his targets doesn't mean he's "good" or even "merciful". Heck, he may even be worse than a High-Chaos Corvo. Fates Worse Than Death, stuff like that. It's well-written and deep. If you've played the game, you're going to enjoy the heck out of this one. WARNING: Low-Chaos Ending Spoilers
http://www.fanfiction.net/s/8901283/1/Diplomatic-Gestures
I never actually thought Corvo as a mute, but it's sure as hell an interesting way of presenting him. If you're interested in filling that empty gap in Dishonored's backstory, you better give this one a try.
http://www.fanfiction.net/s/8628088/1/A-Toast
Have you ever wondered how the toast between Martin and Treavor went in the Low-Chaos Ending? These are spoilers, of course.
It's short, dark and grim. It fits the game's theme perfectly.