I once played to chapter 3 with an assassin, when I noticed my dex, str or con didn't go up with levels. I had quite a low str and con, so I started the game all over again, and rolled a good character (didn't know I could roll in the beginning). Anyway, this is beside the point. At my first game I got this sword of backstabbing, and when I played the other game as a mage, I didn't find it. I can't find it now when I am a sorcerer either. Does anyone know where it is?

Spoiler.......
.
.
.
.
.
.
..
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..
.
.
.
.
.
.
.
.
.
.
Location: Kill Mae'Var, at the end of the Renal Bloodscalp quest.

I believe the sword you are talking about is a short sword. This was on Mae'Var (sp?) who is in the Thieves Guild. I whacked him the first time through too and picked this up not realizing it was +3. Came in pretty handy for awhile.